Immunologic transfusion reactions

Immunologic transfusion reactions are adverse responses in which the recipient’s immune system recognises foreign antigens in a transfused blood component. The antigens may sit on red cells, white cells, platelets or plasma proteins, and the response may be antibody-mediated, complement-mediated, cytokine-driven or neutrophil-driven. This definition matters because the clinical picture depends less on the volume transfused than on the match between donor antigens and recipient immune memory.

A useful first-principles approach is to ask three questions. First, what is the target antigen? Second, is the reaction immediate or delayed? Third, is the main injury intravascular, extravascular or pulmonary? Haemolytic reactions target red cell antigens. Febrile non-haemolytic reactions often involve antibodies against white cell or HLA antigens and cytokines accumulated during storage. Allergic reactions involve soluble allergens and mast cell mediators. Transfusion-related acute lung injury involves priming of pulmonary endothelium and neutrophils, often with donor antibodies or biological response modifiers.

The major immunologic patterns are allergic and anaphylactic reactions, acute and delayed haemolytic reactions, febrile non-haemolytic reactions and transfusion-related acute lung injury. They overlap in early features such as fever, rigors, dyspnoea or hypotension, so the first task is to stop the transfusion and assess the patient while the blood bank investigates. The risk of a fatal haemolytic transfusion reaction is estimated to be 1 in 100,000^[1]. That figure is small in absolute terms but important because haemolysis is largely preventable through correct identification and compatibility testing.

Transfusion-related acute lung injury is a severe adverse event and a leading cause of transfusion-associated death^[1]. It is therefore not enough to think only of fever and rash. Pulmonary injury can occur during or shortly after transfusion and can progress to hypoxia with non-cardiogenic pulmonary oedema. The sections below separate the syndromes so that their mechanisms, timing and management can be learned clearly.

Allergic transfusion reactions arise when donor plasma proteins or other soluble constituents bind to IgE on recipient mast cells and basophils. Cross-linking of IgE triggers degranulation and release of histamine, leukotrienes and other mediators. The result is vasodilatation, increased capillary permeability, smooth muscle contraction and mucus secretion. This is a type I hypersensitivity pattern, and it explains why symptoms can begin within minutes of starting a transfusion and why small amounts of a component can sometimes provoke marked symptoms.

Mild allergic reactions are dominated by cutaneous signs: urticaria, pruritus, flushing and sometimes mild angio-oedema. The patient is usually haemodynamically stable and has no fever or evidence of haemolysis. Anaphylaxis is the severe end of the same spectrum. It produces bronchospasm, laryngeal oedema, hypotension, tachycardia, nausea, vomiting, abdominal cramps and a sense of impending doom. The presence of respiratory compromise or circulatory collapse separates anaphylaxis from a simple urticarial reaction and demands immediate resuscitation.

A key first-principles point is that allergic reactions are not caused by red cell incompatibility. They are therefore not predicted by ABO compatibility alone. Donor plasma is the usual source of the allergen, which is why reactions can occur with plasma-rich components and why some patients react repeatedly. IgA deficiency with anti-IgA antibodies is a classic example of a recipient who can mount a severe reaction to IgA in donor plasma, but many other plasma proteins and drugs can also act as allergens.

Management begins with stopping the transfusion and maintaining the airway, breathing and circulation. For mild urticaria, an antihistamine may relieve symptoms while the patient is observed. For anaphylaxis, adrenaline is the key drug because it reverses vasodilatation, reduces oedema and bronchodilates; antihistamines and corticosteroids do not replace it. Oxygen, fluids and airway support are used according to the clinical state. The transfusion service should be informed so that the component can be investigated and future products can be selected appropriately.

Haemolytic transfusion reactions occur when recipient antibodies bind to donor red cell antigens and cause red cell destruction. In acute haemolytic reactions, preformed antibodies, classically against ABO antigens, bind to donor red cells and activate complement. This produces intravascular haemolysis, release of free haemoglobin and a systemic inflammatory response. The patient may develop fever, chills, chest or back pain, nausea, hypotension, haemoglobinuria and disseminated intravascular coagulation. Renal injury can follow because free haemoglobin and inflammatory mediators impair renal perfusion and tubular function.

Acute haemolysis is a medical emergency. The transfusion must stop at once, the line should be kept open with crystalloid, and the patient should be resuscitated. Blood and urine samples are sent to confirm haemolysis and to investigate compatibility. The blood bank reviews clerical records, labels and crossmatch results because many severe acute haemolytic reactions reflect misidentification at the bedside or in the laboratory. Supportive care includes maintaining blood pressure, correcting coagulopathy and protecting renal function. The risk of a fatal haemolytic transfusion reaction is estimated to be 1 in 100,000^[1].

Delayed haemolytic reactions have a different tempo. They occur when a recipient has been sensitised by previous transfusion or pregnancy, and the antibody titre has fallen below the detection threshold at the time of crossmatch. After transfusion, an anamnestic immune response produces new antibody that binds to donor red cells. Destruction is usually extravascular, in the spleen and liver, so haemoglobinuria is less prominent. The patient may develop fever, jaundice, a falling haemoglobin and a positive direct antiglobulin test several days to weeks after transfusion.

The first-principles distinction is therefore intravascular versus extravascular haemolysis. Acute ABO-incompatible reactions are complement-driven and intravascular, with rapid clinical deterioration. Delayed reactions are often antibody-driven and extravascular, with anaemia and jaundice rather than shock. Both require blood bank investigation, and both illustrate why previous transfusion and pregnancy histories are important when selecting blood.

Febrile non-haemolytic transfusion reactions are characterised by fever and rigors without red cell haemolysis. They arise from two broad mechanisms. First, recipient antibodies may react with donor white cell or HLA antigens, releasing cytokines. Second, bioactive cytokines accumulate in stored blood components and produce a febrile response when transfused. This explains why the reaction can occur without any red cell incompatibility and why it is more closely linked to leucocytes and storage than to ABO mismatch.

Clinically, the patient develops fever, chills, rigors, flushing and discomfort during or shortly after transfusion. The diagnosis is one of pattern recognition and exclusion: haemolysis, bacterial contamination, pulmonary injury and other causes of fever must be considered. A febrile reaction alone should not be assumed to be benign until the transfusion has been stopped and the patient assessed. The blood bank should be notified, and the component should be inspected for discolouration or clots.

Leukoreduction reduces the risk of febrile non-haemolytic transfusion reactions. There is enough evidence to support the contention that transfusion of WBC depleted allogeneic blood decreases the risk for febrile nonhemolytic transfusion reactions (FNHTRs), cytomegalovirus (CMV) transmission, and platelet refractoriness due to HLA alloimmunization^[1]. Universal leukoreduction has been instituted in many countries and has been recommended by an advisory committee to the US Food and Drug Administration in the USA; it decreases the rate of FNHTRs, the risk of CMV transmission, and the unresponsiveness of platelet transfusion after previous alloimmunization^[1].

Management is supportive. The transfusion is stopped, the patient is examined, and antipyretics may be used once serious causes have been considered. Future transfusions may be given with leucodepleted components where available. The key teaching point is that fever after transfusion is never automatically a “simple febrile reaction”; it is a signal to pause and exclude haemolysis, sepsis and pulmonary injury.

Transfusion-related acute lung injury is a rare but severe adverse event that occurs during—or within—6 hours of blood transfusion and is characterized by hypoxia and non-cardiogenic pulmonary edema^[1]. The patient develops respiratory distress, hypoxaemia and bilateral pulmonary infiltrates without evidence that hydrostatic pulmonary oedema is the main cause. It is a leading cause of transfusion-associated death^[1], and its poor prognosis is due in large part to the current dearth of effective therapeutic strategies^[1].

The classification has been refined. A newer diagnostic approach using the Delphi method subdivides TRALI into Type I and Type II, depending on the absence or presence of acute respiratory distress syndrome (ARDS) risk factors^[1]. In the redefinition, the term “possible TRALI” has been dropped^[1]. The terminology of TRALI Type I (without an ARDS risk factor) and TRALI Type II (with an ARDS risk factor or with mild existing ARDS) is proposed^[1]. Cases with an ARDS risk factor that meet ARDS diagnostic criteria and where respiratory deterioration over the 12 hours before transfusion implicates the risk factor as causative should be classified as ARDS^[1].

TRALI remains a clinical diagnosis and does not require detection of cognate white blood cell antibodies^[1]. This is important because antibody testing may be negative in genuine cases, and management should not wait for specialised serology. The time frame in which TRALI could occur should not change from the original definition, that is, within 6 hours after transfusion^[1]. The 6‐hour time frame refers to the occurrence of respiratory compromise or hypoxemia^[1]. The additional findings needed to diagnose TRALI, namely pulmonary oedema on lung imaging and determination of lack of significant left atrial hypertension, would ideally be available at the same time but could be documented up to 24 hours after TRALI onset^[1].

The mechanism is best understood through a priming model followed by a pulmonary reaction and an effector phase^[1]. During the priming step, recipient-related risk factors mainly lead to endothelial activation and PMN priming via the EC-PMN interaction^[1]. In the pulmonary reaction, antibodies or biological response modifiers bind to target cells such as endothelial cells, neutrophils or mononuclear cells, inducing a host response^[1]. In the effector phase, the pulmonary vascular endothelial barrier may become damaged by the release of neutrophil extracellular traps and reactive oxygen species, and the resulting coagulopathy might aggravate lung injury^[1].

Management is largely supportive because specific treatments are still lacking^[1]. Oxygen and ventilatory support are used according to the severity of hypoxaemia, and fluid balance must be managed carefully because increased pulmonary vascular permeability worsens oedema when hydrostatic pressures rise. Prevention is central: risk mitigation for plasma-associated and platelet-associated TRALI has been implemented in many services. Clinical services should report all cases of posttransfusion (within 6 hr) pulmonary edema to the transfusion service even if ARDS risk factors were present and the clinician believed the case to be ARDS^[1].

The timing of an immunologic transfusion reaction is one of the most useful diagnostic clues. Allergic reactions usually begin within minutes of starting the transfusion, and anaphylaxis can deteriorate rapidly over the same period. Acute haemolytic reactions also occur minutes to hours after the start of transfusion, because preformed antibodies can bind immediately to donor red cells. Febrile non-haemolytic reactions typically occur during or within a few hours after transfusion, as cytokines and antibody-antigen interactions provoke fever and rigors.

TRALI has a distinctive window. It occurs during—or within—6 hours of blood transfusion^[1]. The 6‐hour time frame refers to the occurrence of respiratory compromise or hypoxemia^[1]. The additional findings needed to diagnose TRALI could be documented up to 24 hours after TRALI onset^[1], which means that the clinical onset is the key anchor, not the time at which every investigation is completed. A patient who develops ARDS within 6 hours of transfusion in the presence of respiratory deterioration in the 12 hours before transfusion should be classified as ARDS^[1]. This distinction prevents every post-transfusion pulmonary deterioration from being labelled TRALI when an alternative ARDS driver is more likely.

Delayed haemolytic reactions occur days to weeks after transfusion. The delay reflects the time needed for an anamnestic antibody response to produce enough antibody to destroy donor red cells. This contrasts with acute haemolysis, where antibody is already present. The time course therefore points to the underlying immune mechanism: immediate reactions usually reflect preformed antibody or mast cell triggering, whereas delayed reactions reflect immune memory and a new antibody response.

A practical timeline helps at the bedside. If symptoms begin during the first minutes, consider anaphylaxis, acute haemolysis or a vasovagal reaction. If fever develops during or shortly after transfusion, consider febrile non-haemolytic reaction, haemolysis or infection. If respiratory distress develops within the first 6 hours, consider TRALI and fluid overload. If anaemia and jaundice appear days later, consider delayed haemolysis. The timeline does not replace investigation, but it narrows the differential quickly.

The mechanisms of immunologic transfusion reactions can be grouped into antibody-mediated red cell destruction, mast cell-mediated allergy, cytokine-mediated fever and neutrophil-mediated pulmonary injury. In haemolysis, recipient antibody binds donor red cell antigen; complement activation and Fc receptor engagement lead to intravascular or extravascular clearance. In allergy, allergen cross-links IgE on mast cells, releasing histamine and other mediators. In febrile non-haemolytic reactions, HLA or white cell antibodies and stored cytokines produce fever. In TRALI, priming of neutrophils and pulmonary endothelium is followed by antibody or biological response modifier binding and then effector injury.

The TRALI model illustrates how a transfusion reaction can be understood from first principles. The recipient’s underlying condition provides a priming signal. The donor component provides antibodies or biological response modifiers that bind to target cells. Activated neutrophils, endothelial cells, monocytes and platelets release neutrophil extracellular traps and reactive oxygen species, which damage the pulmonary vascular endothelium and produce oedema^[1]. Coagulation disturbance can amplify the injury. This is why TRALI is a pulmonary microvascular syndrome rather than a simple antibody-antigen reaction on red cells.

Management follows a common sequence. Stop the transfusion immediately, keep the intravenous line patent with crystalloid, assess airway, breathing and circulation, and call for senior help when the patient is unstable. Take blood and urine samples for haemolysis, send the component and giving set to the blood bank, and check the identity of the patient and the unit. Treat anaphylaxis with adrenaline and supportive care. Treat acute haemolysis with fluids, blood pressure support and management of coagulopathy. Treat TRALI with oxygen, ventilatory support and careful fluid management. Specific treatments for TRALI are still lacking^[1].

Prevention is part of management. Leukoreduction decreases the rate of FNHTRs, the risk of CMV transmission, and the unresponsiveness of platelet transfusion after previous alloimmunization^[1]. Blood services use donor selection, antibody screening and plasma risk mitigation to reduce TRALI. Clinical teams can prevent haemolysis by strict bedside identity checks and by investigating any discrepancy before transfusion. They can prevent avoidable allergic and febrile reactions by documenting previous reactions and selecting appropriate components.

Immunologic transfusion reactions range from common nuisance symptoms to rare life-threatening events. The frequency of each syndrome depends on the component transfused, the recipient’s immune history and the sensitivity of the reporting system. Fatal haemolytic transfusion reactions are estimated to occur at a rate of 1 in 100,000^[1]. This low rate reflects improvements in compatibility testing and bedside identification, but the consequence of an error can be catastrophic. Febrile non-haemolytic reactions and mild allergic reactions are much more common in clinical practice, although reporting is variable and many mild cases are managed without formal hemovigilance notification.

TRALI is a leading cause of transfusion-associated morbidity and mortality in most developed countries^[1]. It is rare but severe, and its recognition depends on clinical vigilance because there is no single diagnostic laboratory test. In one study, TRALI was observed postoperatively in 2.4% of cardiac surgery patients^[1]. That figure illustrates how a high-risk clinical population with inflammation, cardiopulmonary bypass and multiple transfusions can have a higher observed incidence than the general transfused population. It also shows why incidence estimates cannot be transferred directly from one clinical setting to another.

Epidemiological data must be interpreted with care. Case reporting does not always include all of the needed clinical and laboratory data to accurately assess the role of transfusion^[1]. Under-reporting of mild allergic and febrile reactions means that hospital-based figures may underestimate their true frequency. Conversely, severe reactions such as TRALI and acute haemolysis are more likely to be investigated and reported. The epidemiology therefore describes the visible tip of a broader clinical spectrum, and comparisons between hospitals should take reporting practice into account. The apparent incidence also depends on whether surveillance is passive or active, and on whether reactions that begin after the patient leaves the transfusion area are captured.

Surveillance also shapes prevention. Hemovigilance systems compare case findings to diagnostic criteria, grade severity and assign imputability. The shift from “possible TRALI” to TRALI Type I and Type II was intended to reduce ambiguity and improve case classification. Better classification allows blood services to identify donor and product risk factors and to monitor the effect of risk-reduction strategies. For clinicians, the practical lesson is that every suspected reaction deserves documentation, because aggregated reports are what reveal trends and trigger changes in blood component selection.

The aetiology of immunologic transfusion reactions depends on the interaction between recipient immune status, donor antigens and the component transfused. In haemolysis, the key risk factor is incompatibility between recipient antibody and donor red cell antigen, often through misidentification or an unrecognised alloantibody. In allergic reactions, the risk relates to recipient IgE against plasma proteins or other constituents, and to the amount of allergen in the component. In febrile non-haemolytic reactions, previous sensitisation to HLA or white cell antigens and the leucocyte content of the component are important.

TRALI has well-described patient and transfusion risk factors. Mechanical ventilation causes lung inflammation, and TRALI risk was also associated with pretransfusion peak airway pressure of more than 30 cm H2O^[1]. Chronic alcohol abuse and current smoking are other existing patient inflammatory conditions identified as TRALI risk factors in multivariate analysis after controlling for transfusion and other patient risk factors^[1]. Larger positive fluid balance plays a role because it increases hydrostatic pressure and lowers the threshold for developing pulmonary oedema^[1].

Transfusion-related risk factors include donor antibodies and the volume and strength of the plasma product. The cognate antibody strength and the volume of plasma containing the antibody are contributing factors; transfusion of larger volumes of HNA antibody and larger volumes of strong (e.g., MFI > 1500) cognate HLA Class II antibodies increase the risk for TRALI^[1]. Plasma-rich products such as fresh-frozen plasma were associated with higher risk than red cell units in an international study conducted before a male-predominant plasma transfusion strategy was in widespread use^[1].

Clinical scenarios can combine several risk factors. Major trauma can present with shock, lung contusion and massive transfusion, all of which induce inflammation^[1]. Cardiac surgery patients may be exposed to inflammation, high airway pressure while ventilated and many transfusions; time on pump was an independent TRALI risk factor, explained by the neutrophil-priming capacity of the cardiopulmonary bypass circuit^[1]. Massive transfusion in the 48 hours before onset of pulmonary oedema resulted in an increased risk of developing ARDS after transfusion, after a subsequent transfusion of additional blood units^[1].

Prevention therefore targets both donor and recipient factors. Male-predominant plasma strategies, antibody screening and leucodepletion reduce the burden of immunological mediators. Clinicians can reduce risk by avoiding unnecessary transfusion, using restrictive indications and paying attention to fluid balance and ventilatory pressures in vulnerable patients.

Immunologic transfusion reactions begin with recognition of foreign antigen by the recipient’s immune system. The antigen may be on red cells, leucocytes, platelets or plasma proteins. The response may be antibody-mediated, complement-mediated, cytokine-driven or neutrophil-driven. A useful first-principles model is to separate the target cell from the effector mechanism. Red cell antigens lead to haemolysis. Leucocyte and HLA antigens lead to febrile reactions and, in some settings, platelet refractoriness. Plasma proteins lead to allergic reactions. Pulmonary endothelial and neutrophil activation leads to transfusion-related acute lung injury.

In transfusion-related acute lung injury, the pathophysiology is best understood as a priming step followed by a pulmonary reaction and an effector phase. Recipient factors such as inflammation, surgery, trauma, mechanical ventilation or sepsis prime neutrophils and activate pulmonary endothelium. The transfused component then provides antibodies or biological response modifiers that bind to target cells such as endothelial cells, neutrophils or mononuclear cells. This binding triggers a host response in the lung. Activated effector cells release neutrophil extracellular traps and reactive oxygen species, which damage the pulmonary vascular endothelium and increase permeability. Coagulation and fibrinolysis dysregulation can amplify the injury, and microthrombosis has been observed in pulmonary vascular walls in experimental models^[1].

Antibody-dependent mechanisms are estimated to be responsible for 50–80% of TRALI cases^[1]. Anti-HLA and anti-HNA antibodies are the classic examples, but cases meeting diagnostic criteria have also been reported after transfusion of products without detectable antibodies^[1]. This observation supports the idea that biological response modifiers and other non-antibody mechanisms can also produce the syndrome. The pathophysiology is therefore not a single antibody-antigen reaction but a spectrum of immune and inflammatory pathways converging on pulmonary capillary leak.

For allergic reactions, the first-principles mechanism is mast cell and basophil degranulation after allergen cross-links IgE. Mediators such as histamine increase vascular permeability, dilate vessels and contract bronchial smooth muscle. For febrile non-haemolytic reactions, recipient antibodies against donor leucocyte or HLA antigens and cytokines accumulated during storage produce fever and rigors without red cell destruction. For haemolysis, recipient antibodies bind donor red cells, activate complement and lead to intravascular or extravascular clearance. The common theme is immune recognition, but the target and effector pathway determine the clinical syndrome.

Immunologic transfusion reactions are classified by immune mechanism, target antigen and timing. Allergic reactions are immediate hypersensitivity reactions, ranging from mild urticaria to anaphylaxis. Haemolytic reactions are classified as acute or delayed according to the tempo of red cell destruction. Febrile non-haemolytic reactions are classified by the absence of haemolysis and by their association with leucocytes and stored cytokines. Transfusion-related acute lung injury is classified by its pulmonary presentation and its relationship to ARDS risk factors.

The TRALI classification has been revised. In the redefinition, the term “possible TRALI” has been dropped^[1]. The terminology of TRALI Type I and TRALI Type II is proposed, with Type I occurring without an ARDS risk factor and Type II occurring with an ARDS risk factor or with existing mild ARDS^[1]. This classification preserves the clinical term TRALI while incorporating the modern understanding of ARDS risk factors.

For a case to be classified as TRALI Type II, the case must meet three conditions^[1]. First, it must fulfil the same clinical criteria as TRALI Type I. Second, the onset of posttransfusion pulmonary oedema must occur in the presence of an ARDS risk factor or mild ARDS^[1]. Third, there must have been a stable pulmonary status in the 12 hours before transfusion^[1]. If the first two criteria are met but the third is not, the case should be classified as ARDS unless documentation that the P/F ratio is less than 300 is lacking, in which case it should be classified as transfusion-associated dyspnoea^[1].

A patient with an ARDS risk factor who shows no respiratory deterioration in the 12 hours before transfusion and who then deteriorates after transfusion should be classified as having TRALI and further subclassified as TRALI Type II^[1]. This criterion is clinically useful because it distinguishes a transfusion-triggered deterioration from a deterioration already underway before the transfusion. In addition, mild pretransfusion ALI, that is mild ARDS, does not rule out a subsequent diagnosis of TRALI^[1]. This represents an expansion compared with older definitions.

Other immunologic reactions do not have a formal staging system. They are classified descriptively: allergic versus anaphylactic, acute versus delayed haemolytic, febrile non-haemolytic, and TRALI Type I versus Type II. The classification matters because it guides investigation and prevention. For example, a febrile reaction requires exclusion of haemolysis and infection, whereas a pulmonary reaction requires distinction from circulatory overload and ARDS.

The clinical features of immunologic transfusion reactions reflect the target antigen and the effector pathway. Allergic reactions produce urticaria, pruritus, flushing and sometimes angio-oedema. Anaphylaxis adds bronchospasm, laryngeal oedema, hypotension, tachycardia, nausea, vomiting and abdominal cramps. The patient may report a sense of impending doom. The presence of respiratory compromise or circulatory collapse is the key distinction between a mild allergic reaction and anaphylaxis.

Acute haemolytic reactions produce fever, chills, chest or back pain, nausea, hypotension, haemoglobinuria and a systemic inflammatory response. The patient may become shocked, and disseminated intravascular coagulation can develop. Delayed haemolytic reactions are more insidious, with fever, jaundice, a falling haemoglobin and a positive direct antiglobulin test days to weeks after transfusion. Febrile non-haemolytic reactions produce fever, rigors, chills and flushing during or shortly after transfusion, without evidence of haemolysis.

Transfusion-related acute lung injury presents with respiratory distress, hypoxaemia and bilateral pulmonary infiltrates. The syndrome occurs during or within 6 hours of transfusion and is characterised by hypoxia and non-cardiogenic pulmonary oedema^[1]. The patient may have dyspnoea, tachypnoea, cyanosis and a fall in oxygen saturation. The pulmonary oedema is non-cardiogenic, meaning that increased capillary permeability, rather than hydrostatic pressure alone, is the primary mechanism. This is why the patient may not respond fully to diuresis and why fluid administration can worsen the pulmonary picture.

The clinical features overlap. A patient with TRALI may be febrile, and a patient with acute haemolysis may become breathless. A patient with anaphylaxis may develop pulmonary oedema. For this reason, the history, timing, examination and simple investigations must be interpreted together. The presence of hypotension with fever and haemoglobinuria points towards haemolysis. The presence of wheeze and urticaria points towards allergy. The presence of bilateral infiltrates with hypoxia after transfusion points towards TRALI or circulatory overload.

The differential diagnosis of immunologic transfusion reactions includes non-immunologic transfusion complications and other acute medical conditions. For pulmonary symptoms, the main distinction is between TRALI and transfusion-associated circulatory overload. Both are characterised by respiratory distress due to onset of pulmonary oedema within 6 hours of transfusion^[1]. However, the primary mechanism differs. In TRALI, the primary physiologic abnormality is an increase in capillary permeability, whereas in TACO, oedema is believed to result primarily from increased capillary hydrostatic pressure, that is, non-permeability oedema^[1].

The published ISBT diagnostic criteria for TACO, which have recently undergone revision and field validation, also include tachycardia and increased blood pressure^[1]. These features can help at the bedside, although they are not absolute. Echocardiography can be useful when it has been ordered, because it may help distinguish permeability oedema from hydrostatic oedema^[1]. In practice, the distinction is not always possible, and cases may be classified as TRALI and TACO cannot be distinguished^[1].

Transfusion-transmitted bacterial infection or an anaphylactic reaction should be considered in the differential diagnosis of posttransfusion pulmonary oedema^[1]. Bacterial contamination can produce fever, hypotension and respiratory distress, and anaphylaxis can produce bronchospasm and pulmonary oedema. Other causes of fever after transfusion include acute haemolysis, sepsis and a febrile non-haemolytic reaction. Other causes of breathlessness include fluid overload, underlying cardiac disease, pulmonary embolism and progression of the patient’s primary lung disease.

The differential diagnosis of haemolysis includes a delayed haemolytic reaction, a febrile non-haemolytic reaction and sepsis. Haemoglobinuria is a strong clue to intravascular haemolysis, but it may be absent in extravascular delayed haemolysis. The differential diagnosis of allergic reactions includes anaphylaxis, vasovagal reactions and immunization stress-related responses, although the latter are more relevant to vaccination than transfusion. A vasovagal reaction produces pallor, sweating, bradycardia and hypotension, but not urticaria or wheeze.

The investigation of a suspected immunologic transfusion reaction starts with the bedside assessment and the transfusion history. The timing of symptoms, the component transfused, the volume given and the patient’s previous reactions all shape the differential diagnosis. The transfusion should be stopped while samples and records are checked, because acute haemolysis and anaphylaxis can deteriorate quickly. A structured review of the patient, the unit and the giving set is more useful than any single laboratory test.

For pulmonary reactions, the main task is to distinguish permeability oedema from hydrostatic oedema and to exclude other causes of respiratory distress. Clinical examination, fluid balance, chest imaging and cardiac assessment are used together. Echocardiography and natriuretic peptide measurement may help when available, but the diagnosis is not made from one test alone. A patient who develops hypoxia and bilateral infiltrates after transfusion requires careful review of the timing of onset and of any pre-existing respiratory disease.

For suspected haemolysis, basic investigations include a full blood count, reticulocyte count, lactate dehydrogenase, bilirubin, haptoglobin, direct antiglobulin test, blood film and urine examination for haemoglobin. These tests show whether red cells are being destroyed and whether the process is intravascular or extravascular. The blood bank reviews clerical records, labels and crossmatch results, because misidentification remains an important cause of severe haemolysis. Antibody identification may take time, but immediate supportive care should not wait for it.

For suspected allergic or anaphylactic reactions, the diagnosis is clinical. Serum tryptase may be useful in some settings but is not a substitute for immediate treatment. For suspected febrile non-haemolytic reactions, the main task is to exclude haemolysis and infection. For suspected circulatory overload, clinical assessment of fluid balance and cardiac function is central. Suspected cases should be reported to the transfusion service so that the component and the clinical timeline can be reviewed and the case can be classified for hemovigilance.

The diagnostic criteria for TRALI have been refined to reduce ambiguity. TRALI Type I is diagnosed when the clinical criteria for TRALI are met without an ARDS risk factor. TRALI Type II is diagnosed when the same clinical criteria are met in the presence of an ARDS risk factor or mild ARDS^[1]. For Type II, there must also have been a stable pulmonary status in the 12 hours before transfusion^[1].

A patient with an ARDS risk factor who shows no respiratory deterioration in the 12 hours before transfusion and who then deteriorates after transfusion should be classified as TRALI Type II^[1]. If the patient has an ARDS risk factor, meets ARDS diagnostic criteria, and has respiratory deterioration in the 12 hours before transfusion that implicates the risk factor as causative, the case should be classified as ARDS^[1]. The distinction depends on whether the deterioration began before the transfusion or after it.

Mild pretransfusion ALI does not rule out a subsequent diagnosis of TRALI^[1]. This means that a patient with mild existing ARDS can still be classified as TRALI Type II if the other criteria are met. If the case meets the first two Type II conditions but lacks a stable pulmonary status in the 12 hours before transfusion, it should be classified as ARDS unless documentation that the P/F ratio is less than 300 is lacking, in which case it should be classified as transfusion-associated dyspnoea^[1].

TRALI remains a clinical diagnosis and does not require detection of cognate white blood cell antibodies^[1]. There are currently no biomarkers available to diagnose TRALI^[1]. The presence of cognate WBC antibodies should not be a criterion for diagnosis^[1]. This is a key point for students: the diagnosis is made from the clinical picture, timing, imaging and exclusion of alternative causes, not from a single laboratory result.

For other immunologic reactions, diagnostic criteria are largely clinical and laboratory-based. Acute haemolysis is supported by fever, haemoglobinuria, a positive direct antiglobulin test and evidence of red cell destruction. Delayed haemolysis is supported by a falling haemoglobin and a positive direct antiglobulin test days after transfusion. Febrile non-haemolytic reaction is a diagnosis of exclusion once haemolysis and infection are excluded. Allergic reactions are diagnosed from urticaria, pruritus, wheeze or hypotension in temporal relation to transfusion.

The complications of immunologic transfusion reactions depend on the syndrome. Acute haemolysis can cause shock, acute kidney injury, disseminated intravascular coagulation and death. Delayed haemolysis can cause severe anaemia and jaundice. Anaphylaxis can cause airway obstruction, hypotension and cardiac arrest. Febrile non-haemolytic reactions are usually self-limiting but can be distressing and can mask more serious causes of fever. TRALI can cause severe hypoxaemia, prolonged mechanical ventilation and death.

Beyond the immediate reaction, complications can be grouped by organ system. Circulatory complications include hypotension, shock and cardiac arrest. Respiratory complications include bronchospasm, laryngeal oedema, pulmonary oedema and prolonged ventilatory support. Renal complications follow haemolysis and hypoperfusion. Haematological complications include coagulopathy and continuing anaemia. These are not independent; shock can worsen renal injury, and pulmonary injury can worsen hypoxia and cardiac strain.

Patients with TRALI in an intensive care unit exhibited 70% mortality compared with typical 5–25% rates^[1]. This striking difference emphasises that the underlying clinical condition is a major determinant of outcome. Critical illness provides a strong first hit, and such patients may require only a minor second hit to trigger TRALI^[1]. The complication is therefore not merely a pulmonary reaction but a marker of a vulnerable host response.

Coagulopathy may contribute to deteriorated lung function after transfusion of pathogenic antibodies or biological response modifiers that damage pulmonary endothelial cells^[1]. Microthrombosis has been observed in the pulmonary vascular walls of animals with TRALI^[1]. Coagulopathy has been identified as a pivotal pathogenic characteristic in rat models of aged erythrocyte- or platelet-mediated TRALI^[1]. In cardiovascular surgery patients with obvious pulmonary inflammatory reactions, transfusion was accompanied by coagulation and fibrinolysis dysregulation, leading to worsened prognosis in the intensive care unit^[1].

Other complications include transfusion-associated circulatory overload, which can coexist with TRALI and worsen respiratory failure. Cases may be classified as TRALI and TACO cannot be distinguished when data are insufficient or when both occur together^[1]. Bacterial contamination and anaphylaxis can complicate the picture and must be considered in the differential diagnosis of posttransfusion pulmonary oedema^[1]. The broader lesson is that a transfusion reaction can be the start of a systemic illness rather than a transient bedside event.

The prognosis of immunologic transfusion reactions is highly variable. Mild allergic reactions and febrile non-haemolytic reactions usually resolve without long-term sequelae once the transfusion is stopped and supportive care is given. Acute haemolysis and anaphylaxis can be fatal if not recognised and treated promptly. Delayed haemolysis is often self-limiting but can cause significant anaemia. TRALI carries a poor prognosis in critically ill patients, with the mortality figures reflecting the severity of the underlying condition as well as the pulmonary injury.

Patients with TRALI in an intensive care unit exhibited 70% mortality compared with typical 5–25% rates^[1]. This means that prognosis is not determined by the transfusion alone. The presence of underlying disease, the number of organs supported and the degree of inflammation all influence outcome. The first hit is often the dominant driver, and the transfusion is the precipitant that reveals the vulnerability of the pulmonary circulation. Early recognition and supportive care remain the most reliable ways to limit deterioration.

Prevention can improve prognosis. Solvent/detergent-treated pooled plasma was introduced in Norway in 1993, and no TRALI cases were reported over a 10-year period^[1]. As of 2020, no TRALI cases had been reported in association with solvent/detergent-treated pooled plasma transfusion^[1]. However, in 2022, a definite TRALI case was observed after such transfusion, resulting in a total of three reported cases to date^[1]. This shows that risk reduction is substantial but not absolute.

For patients who recover, documentation and communication with the transfusion service are essential before any future transfusion. The component, timing and symptoms should be recorded so that future products can be selected appropriately. Where a donor antibody is implicated, donor information may inform future eligibility decisions. The broader prognostic message is that immunologic transfusion reactions are often preventable, and that prompt recognition, supportive care and avoidance of unnecessary transfusion remain the most dependable ways to improve outcome.

Prevention of immunologic transfusion reactions begins with a simple principle: use blood only when the expected benefit outweighs the risk. Patient blood management, restrictive transfusion thresholds and careful assessment of alternative treatments reduce exposure to donor antigens, leucocytes, plasma proteins and biological response modifiers. This is particularly important in critical care and surgical practice, where transfusion remains common^[1]. Every unit avoided removes the possibility of an allergic, febrile, haemolytic or pulmonary reaction.

Screening for immunologic risk has two arms. The first is recipient assessment: previous transfusion reactions, known alloantibodies, pregnancy history and underlying inflammatory conditions should be documented before blood is requested. The second is donor and component selection: blood services use donor management, blood component processing and patient blood management to reduce transfusion-related morbidity^[1]. These measures are not interchangeable, and they target different steps in the reaction pathway.

Leucoreduction is a central preventive technology. A unit of donated blood is usually fractionated into red cells, buffy coat and plasma. Buffy-coat-free red cells have a 75% reduction in WBCs^[1]. Leucocyte filters remove 99.9% of WBCs^[1]. This matters because leucocytes and their stored mediators contribute to febrile non-haemolytic reactions, HLA alloimmunisation and platelet refractoriness. Leucoreduction has proven benefits for febrile non-haemolytic reactions, cytomegalovirus transmission and platelet refractoriness due to HLA alloimmunisation^[1]. It has not been shown that cancer recurrence is decreased by leucoreduction, and there is not enough evidence to support universal leucoreduction based solely on mortality or postoperative infection effects^[1].

For transfusion-related acute lung injury, prevention targets donor antibodies and plasma-rich components. Anti-HLA or anti-HNA antibodies are detected most frequently in multiparous donors, and risk-reduction strategies including the introduction of male-only donors, male-dominated plasma, exclusion of all-exposure donors, and antibody screening have been introduced^[1]. These strategies have resulted in a significant reduction in the morbidity associated with antibody-dependent TRALI in developed countries^[1]. Anti-leukocyte antibody screening has not yet been routinely implemented because of financial complexities^[1]. The effect of component processing on TRALI incidence should continue to be monitored through hemovigilance.

Screening also has a moving target. Anti-HLA antibodies were reportedly triggered in coronavirus disease 19 convalescent plasma donors and SARS-CoV-2 vaccination volunteers, so TRALI prevention may also require screening antibodies against HLA antigens in donors following SARS-CoV-2 sensitization^[1]. This illustrates why prevention is not a fixed checklist. Donor populations, component processing and recipient vulnerability change over time, and surveillance must adapt. The practical message for students is that prevention combines bedside stewardship, correct identification, component selection and hemovigilance.

Follow-up after a suspected immunologic transfusion reaction has three aims: confirm the diagnosis, document the reaction and prevent recurrence. The immediate clinical follow-up depends on the syndrome. A patient with an allergic reaction is observed for progression to anaphylaxis. A patient with haemolysis is monitored for continuing red cell destruction, renal injury and coagulopathy. A patient with pulmonary symptoms is monitored for hypoxia, ventilatory requirement and fluid balance. A patient with a febrile reaction is monitored for haemolysis and infection.

The blood bank follow-up is equally important. For cases classified as TRALI, the detection of donor WBC antibodies that correspond to recipient cognate antigens is not always a hemovigilance system reporting requirement, but collection of such information is important to identify those reactions with a serologic basis versus those not associated with WBC antibodies^[1]. Additional information about the WBC antibodies, such as detection method and titre, may be valuable for research, although the panel recognises that this may be difficult^[1]. This is a key follow-up principle: the bedside diagnosis does not depend on antibody results, but antibody information improves understanding and future donor selection.

Follow-up also includes classification. The detailed classification system differs from current hemovigilance practice because it includes more categories and incorporates imputability assessment. Its feasibility should be evaluated by hemovigilance experts, perhaps by prospectively comparing accurate classification using the redefinition with the current TRALI definition^[1]. This is not administrative pedantry. Without consistent classification, cases of TRALI, TACO, ARDS and transfusion-associated dyspnoea are mixed together, and prevention strategies cannot be evaluated properly.

Clinical follow-up should also address prognosis. Estimates of TRALI mortality vary markedly between 5% and 25%^[1]. They can reach 47% in intensive care and surgical patients^[1]. Even so, specific therapeutics remain unavailable^[1]. This means that follow-up must include supportive care, treatment of the underlying first hit and avoidance of further unnecessary transfusion. For patients who recover, documentation of the reaction and communication with the transfusion service are essential before any future transfusion.

Follow-up after an allergic or febrile reaction includes recording the component, the timing and the symptoms so that future products can be selected appropriately. Follow-up after haemolysis includes repeat compatibility testing and antibody identification. Follow-up after TRALI includes reporting to the transfusion service and, where relevant, donor antibody testing. In all cases, the patient should be told what happened in plain language and what precautions will apply next time. This is part of safe practice, not an optional extra.

Exam questions on immunologic transfusion reactions often test pattern recognition and timing. Allergic reactions produce urticaria, pruritus and flushing; anaphylaxis adds wheeze, stridor, hypotension or collapse. Acute haemolysis produces fever, pain, hypotension and haemoglobinuria. Febrile non-haemolytic reactions produce fever and rigors without haemolysis. TRALI produces hypoxia and non-cardiogenic pulmonary oedema during or within 6 hours of transfusion. Delayed haemolysis produces anaemia and jaundice days to weeks later.

A favourite comparison is TRALI versus transfusion-associated circulatory overload. Both produce respiratory distress and pulmonary oedema within 6 hours, but TRALI is a permeability oedema and TACO is a hydrostatic, non-permeability oedema^[1]. The Berlin definition dropped the pulmonary capillary wedge pressure criterion in excluding circulatory overload; laboratory assays such as BNP, ultrasound and clinical variables such as fluid balance are used instead^[1]. This is a modern exam point because pulmonary artery catheters are no longer routine.

Another exam pearl concerns the PEEP criterion. The Berlin criterion of using ventilator support, for example at least 5 cmH2O positive end-expiratory pressure treatment, as a necessary criterion to diagnose ARDS is not applicable for diagnosing TRALI^[1]. TRALI may develop on the ward as a mild lung injury in which only supplemental oxygen is needed, so the PEEP criterion is unnecessarily restrictive^[1]. Students should therefore avoid applying every ARDS ventilator criterion rigidly to TRALI.

The two-hit model is frequently examined. An inflammatory condition in the recipient provides the first hit, and the transfusion provides the second hit. Higher concentrations of interleukin-8 in the patient’s pretransfusion plasma is an independent risk factor for TRALI^[1]. This explains why critically ill and injured patients are especially at risk^[1]. Anti-HLA class II antibodies can sensitise peripheral blood mononuclear cells and lead to production of inflammatory cytokines such as IL-1β and IL-6, and chemokines such as IL-8, by monocytes^[1].

Prevention pearls are also testable. Leucocyte filters remove 99.9% of WBCs^[1]. Leucoreduction reduces febrile non-haemolytic reactions, CMV transmission and platelet refractoriness^[1]. Leucoreduction of blood components reduced TRALI cases by 83% in a retrospective analysis^[1]. Male-only donors, male-dominated plasma, exclusion of all-exposure donors and antibody screening are recognised TRALI risk-reduction strategies^[1]. These facts link pathophysiology to prevention, which is a common examination theme.

The first pitfall is assuming that every fever after transfusion is a febrile non-haemolytic reaction. Fever can also occur in acute haemolysis, bacterial contamination and pulmonary injury. A febrile reaction should prompt a pause, a clinical review and notification of the blood bank. The second pitfall is treating urticaria as trivial without checking for airway, breathing or circulatory compromise. Anaphylaxis can begin with skin signs, but the presence of wheeze, stridor, hypotension or collapse changes the management completely.

The third pitfall is diagnosing TRALI from antibody tests alone. TRALI remains a clinical diagnosis, and the presence of cognate WBC antibodies should not be a criterion for diagnosis. There are currently no biomarkers available to diagnose TRALI. Antibody results may take weeks to months, and their presence or absence does not help the bedside physician in the acute setting. The fourth pitfall is failing to distinguish TRALI from TACO. Both produce pulmonary oedema within 6 hours, but the mechanisms differ: TRALI is permeability oedema, whereas TACO is hydrostatic oedema^[1]. Echocardiography, BNP, ultrasound and fluid balance can help when available^[1].

The fifth pitfall is overstating the benefits of leucoreduction. Leucoreduction has three proven benefits, namely febrile non-haemolytic reactions, CMV transmission and platelet refractoriness^[1]. It has not been shown that cancer recurrence is decreased by leucoreduction, and there is not enough evidence to support universal leucoreduction based solely on mortality or postoperative infection effects^[1]. The sixth pitfall is applying the Berlin PEEP criterion rigidly to TRALI. The criterion of at least 5 cmH2O positive end-expiratory pressure as a necessary criterion for ARDS is not applicable for diagnosing TRALI^[1].

The seventh pitfall is forgetting that prevention is not only a laboratory activity. Bedside identity checks, previous reaction history and patient blood management are part of prevention. The eighth pitfall is failing to report cases. Even when the clinician believes the case is ARDS or TACO, reporting allows the transfusion service to classify the case, collect donor antibody information and monitor trends^[1]. Good documentation is therefore part of good clinical care.

Immunologic transfusion reactions are encountered wherever blood is transfused, but they are especially relevant in critical care, surgery, obstetrics, haematology and oncology. In critical care and surgical settings, transfusion remains common and patients often have inflammatory first hits that make TRALI more likely^[1]. In these patients, the bedside team must distinguish transfusion reactions from sepsis, ARDS, fluid overload and progression of the underlying disease.

In the blood bank, prevention and screening are daily activities. Donor selection, male-predominant plasma strategies, antibody screening, leucoreduction and component processing are used to reduce the burden of immunological mediators^[1]. When a reaction occurs, the transfusion service reviews the component, the timing and the clinical data, and classifies the case. This may require dialogue between clinicians and transfusion medicine specialists, and the classification may include TRALI Type I or II, ARDS, TACO, TRALI and TACO cannot be distinguished, transfusion-associated dyspnoea, or an alternate diagnosis^[1].

In hemovigilance, the new classification supports monitoring and research. The detailed system includes more categories and incorporates imputability assessment, so its feasibility should be evaluated prospectively^[1]. Collection of donor WBC antibody information is important to separate serologic from non-serologic TRALI^[1]. This information also informs donor eligibility and future risk-reduction strategies.

On the ward, practice is about early recognition. Stop the transfusion, assess the airway, breathing and circulation, and call for help when the patient is unstable. Check the identity of the patient and the unit. Send the component and giving set to the blood bank. Document the timing and symptoms. For pulmonary reactions, consider TRALI, TACO, bacterial contamination and anaphylaxis. For fever, consider haemolysis and infection. For allergic symptoms, look for airway involvement. These simple actions connect the teaching to real patient safety.