IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 24. Hypersensitivity - Type II

REACTIONS AGAINST BLOOD CELLS

The most striking Examples of Type II hypersensitivity are anti-erythrocyte reactions. They can cause severe consequences in the following situations:

✵ transfusion of incompatible Blood when the recipient is sensitized to the donor's surface erythrocyte Antigens;

✵ hemolytic disease of the newborn, resulting from the sensitization of a pregnant woman by fetal erythrocytes, and

✵ autoimmune hemolytic anemias, when the patient is sensitized to their own erythrocytes.

Anti-platelet reactions can cause thrombocytopenia; in systemic lupus erythematosus, anti-neutrophil and anti-lymphocyte reactions occur.

Transfusion reactions develop when the recipient has Antibodies that react with the donor's erythrocytes

Over 20 blood group systems have been identified in humans, defining the existence of more than 200 genetic variants of erythrocyte antigens. Each blood group system represents a Gene locus that determines the Specificity of antigens On the surface of Blood Cells (usually, but not always, erythrocytes). In each system, two or more phenotypes are possible. The ABO system, for example, determines four possible phenotypes (A, B, AB, and 0) and, consequently, four Blood Groups. Immunocytes of an individual with a given blood group recognize erythrocytes carrying allogeneic (non-self) antigens and produce antibodies against them. For some group antigens, antibodies may also be "natural", i.e., produced without prior sensitization by foreign erythrocytes (see below). Transfusion of allogeneic erythrocytes to a person who has antibodies against them can cause erythrocyte destruction and the appearance of "transfusion reaction" symptoms. Some Blood Group Antigens (ABO and Rhesus) possess relatively strong immunogenicity, meaning they are more likely to induce antibody formation. When planning a blood transfusion, It is important to ensure compatibility between the donor and recipient regarding these major blood groups; otherwise, transfusion reactions will occur. Some of the main human blood group systems are listed in Fig. 24.6.

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Fig. 24.6. Not all group antigens have equal antigenicity: thus, RhD causes a stronger transfusion reaction in an incompatible recipient than other Rhesus system antigens, and Fy^a causes a stronger one than Fy^b. The indicated phenotype frequencies are typical for Caucasians. Other races exhibit different gene frequencies.

The ABO system. This is the most important system; the corresponding epitopes are present not only on erythrocytes but also on many other Cell types and are localized in the carbohydrate portion of Glycoproteins. The Structure of these CARBOHYDRATES, like those defining the closely related Lewis blood group system, depends on The genes of Enzymes that transport terminal sugars to the carbohydrate backbone (Fig. 24.7). Most people have antibodies to allogeneic ABO system antigens, as their production does not require prior sensitization by foreign erythrocytes; the necessary sensitization occurs through contact with identical epitopes expressed on the cells of many microorganisms. Therefore, antibodies to ABO antigens are very common, making matching donor blood specifically by this system extremely important. However, all people are tolerant to the 0 antigen, and therefore carriers of this antigen are universal Donors with respect to the AB0 system.

Fig. 24.7. Diagram illustrating The formation of the ABO blood group system. The enzyme encoded by the H gene attaches a fucose residue (Fuc) to the terminal galactose (Gal) residue of the oligosaccharide precursor. In individuals with the A gene, N-acetylgalactosamine (N-Ac-Gal) is attached to this galactose residue, and in individuals with the B gene, a second galactose molecule is attached, resulting in the formation of antigens A and B, respectively. Individuals with both genes produce both antigens. The table shows the genotypes and antigens of the ABO system. Most people produce "natural" antibodies to the antigens they lack.

The Rhesus system. This system is also very important, as it is associated with the primary cause of hemolytic disease of the newborn. Rhesus antigens are associated with 30 kDa Membrane Proteins that are moderately expressed on The surface of erythrocytes. These antigens are encoded by two closely linked gene loci — RhD and RhCcEe, which share 92% Homology. Clinically, RhD is the most important because it is highly immunogenic; in RhD^- individuals, the RhD locus is completely absent. The RhCcEe locus encodes a molecule containing the RhC/c and RhE/e epitopes.

Minor blood group systems. The epitopes of the MN system are represented by N-terminal glycosylated regions of Glycophorin A, a surface glycoprotein of erythrocytes. Antigenicity is determined by the polymorphism of amino acid residues 1 and 5. Glycophorin B carries the antigens of the closely related Ss system. The dependence of blood groups on erythrocyte surface proteins is presented in Fig. 24.8. Minor blood groups relatively rarely cause transfusion reactions unless repeated transfusions are performed. Once again, careful Selection of donor blood dramatically reduces the risk of such reactions.

Fig. 24.8. Note that group epitopes formed by carbohydrate structures, such as ABO and Ii (expressed on the ABO precursor polysaccharide), can appear on many different proteins, including Rh antigens. Other group antigens, such as Rhesus and Cromer, are proteins, so these epitopes can only be carried by specific protein molecules. As a rule, the most important blood group antigens are present on erythrocytes in large quantities, thereby creating an Abundance of targets for Complement-dependent lysis or Fc receptor-mediated clearance.

Crossmatching. The purpose of this test is to check whether the recipient's blood contains antibodies capable of reacting with and destroying the donor's erythrocytes. For example, antibodies to ABO system antigens cause agglutination of incompatible cells, and this reaction is visible to the naked eye. Antigens of minor blood group systems cause less pronounced reactions that can only be detected using the indirect Coombs test (see Fig. 24.12). When transfusing whole blood, it is also necessary to ensure that the donor's serum does not contain antibodies against the recipient's erythrocytes. However, whole blood transfusions are rarely performed. Donor blood is usually separated into cellular and serum fractions, which are used separately.

Transfusion reactions involve massive destruction of host blood cells. Transfusion of erythrocytes to a recipient who has antibodies against these cells causes an immediate reaction — fever, hypotension, nausea and vomiting, as well as back and chest pain. The severity of the reaction depends on the class and quantity of the antibodies involved.

Antibodies to ABO system antigens usually belong to the IgM class and cause agglutination, complement activation, and intravascular hemolysis.

Other blood group antigen systems induce the formation of IgG antibodies, which agglutinate erythrocytes less effectively compared to IgM. Cells sensitized by IgG are usually engulfed by phagocytes of The Liver and Spleen, although in severe reactions, erythrocyte destruction also occurs due to complement activation. In such cases, circulatory collapse may develop, and the released erythrocyte contents can cause acute renal tubular necrosis. These acute transfusion reactions are often observed in non-sensitized individuals and develop within a few days or weeks as antibodies to foreign cells are produced. As a result, anemia or jaundice occurs.

Transfusion reactions to other blood components are also possible, although their consequences are generally not as serious as those to erythrocytes.

Hyperacute graft rejection is associated with a transfusion reaction. In cases where the recipient already has antibodies against the transplanted tissue, hyperacute graft rejection occurs. This is possible only with Tissues that are revascularized immediately after transplantation, such as a Kidney transplant. The most severe rejection reactions of this type are caused by ABO system antigens present on kidney cells. Damage is caused by antibodies and complement activation in Blood Vessels, followed by the recruitment and activation of neutrophils and platelets. However, nowadays, thanks to careful donor-recipient matching for ABO antigens, such reactions are extremely rare. Antibodies to other graft antigens (e.g., MHC molecules) formed during previous transplants can also cause reactions of this type.

Haemolytic disease of the newborn is caused by maternal IgG antibodies that react with fetal erythrocytes in utero

Haemolytic disease of the newborn (HDN) develops when a mother is sensitised to fetal erythrocyte antigens and produces IgG antibodies against these antigens. Such antibodies cross the Placenta and react with fetal red blood cells, causing their destruction (Figs. 24.9 and 24.10). Most commonly, this reaction involves the Rhesus D (RhD) antigen.

Fig. 24.9. Rhesus-positive (RhD⁺) fetal red blood cells typically enter the maternal Circulation during childbirth. This stimulates The production of anti-Rh IgG antibodies in the postnatal period. During subsequent pregnancies, these IgG antibodies cross the placenta into the fetal bloodstream (IgM antibodies do not cross the placenta). If the fetus is again RhD⁺, the maternal IgG antibodies cause the destruction of its erythrocytes.

Fig. 24.10. An infant suffering from HDN. This condition is characterised by a significant enlargement of the liver and spleen resulting from erythrocyte destruction caused by maternal anti-erythrocyte antibodies that have entered the Fetal circulation. The infant's blood shows an elevated concentration of bilirubin (a haemoglobin breakdown product). Haemorrhagic petechiae on the face are due to impaired platelet function. Most commonly, these reactions are directed against the RhD antigen. (Photograph kindly provided by Dr C. Sloper).

During a second Pregnancy in a sensitised Rh^- mother, if the fetus is once again Rh⁺, the risk of HDN increases. Sensitisation of an Rh^- mother by Rh⁺ fetal erythrocytes typically occurs during the delivery of the first Rh⁺ child, when some of their red blood cells cross the placenta into the maternal circulation and are recognised by her immune system. Thus, the first Rh^--incompatible child generally remains healthy, whereas subsequent children are at high risk of disease because the mother is re-sensitised with each new pregnancy.

HDN may also be associated with reactions to other blood group antigens; the next most frequent immunogenic antigen is the Kell system K antigen. Reactions caused by anti-K antibodies are much less common than those caused by anti-RhD, because the K antigen is relatively rare (9%) and possesses weaker antigenicity.

It is well established that the risk of HDN due to Rhesus incompatibility is reduced if the mother and father have different ABO blood groups. This observation led to the hypothesis that in such cases, Rh⁺ cells are destroyed more rapidly in the Rh^- mother because they are also incompatible for ABO blood group antigens. As a result, the fetal Rh⁺ erythrocytes do not persist long enough to sensitise the mother. This concept led to The Development of Rhesus prophylaxis: immediately after the birth of an Rh⁺ child, the Rh^- mother is administered anti-RhD antibodies to destroy any fetal Rh⁺ erythrocytes that have entered her circulation before they can exert a sensitising effect. This approach has significantly reduced the incidence of HDN caused by Rhesus incompatibility (Fig. 24.11).

Fig. 24.11. 1. In the absence of prophylaxis, Rh⁺ erythrocytes entering the circulation of an Rh^- mother sensitise her to the Rh antigen. 2. If the woman is administered anti-Rh (anti-D) antibodies immediately after childbirth, these antibodies destroy the Rh⁺ red blood cells and prevent sensitisation. Between 1950 and 1966, the decline in mortality from HDN was attributed to general improvements in healthcare. The Introduction of Rhesus prophylaxis in 1969 markedly accelerated this decline.

Autoimmune haemolytic anaemias occur either spontaneously, as autoimmune diseases, or as reactions to medicinal drugs

Reactions to blood group antigens can develop spontaneously, as seen in autoimmune haemolytic anaemias, where patients produce antibodies against their own red blood cells. Autoimmune haemolytic anaemia is suspected when a patient's blood yields a positive result in the direct antiglobulin test (Fig. 24.12), which detects the presence of antibodies on erythrocytes. Typically, these antibodies are directed against erythrocyte antigens or immune complexes adsorbed onto the red cell surface. The indirect antiglobulin test is also used to detect antibodies on red blood cells in mismatched blood transfusions and in HDN (see above). Autoimmune haemolytic anaemias can be divided into three types depending on their underlying cause:

✵ warm autoantibodies, which react with the antigen at 37 °C,

✵ cold autoantibodies, which react with the antigen only at temperatures below 37 °C, or

✵ antibodies generated during an allergic reaction to medicinal drugs.

Fig. 24.12. This test, also known as the Coombs test, is used to detect antibodies on the surface of a patient's red blood cells. In the presence of such antibodies, anti-human immunoglobulin antibodies cause erythrocyte agglutination. In the absence of antibodies on the red cells, human antiglobulin does not cause agglutination.

Warm autoantibodies accelerate the clearance of erythrocytes. Warm autoantibodies are frequently directed against Rhesus system antigens, including determinants of the RhC, RhE, and RhD loci. They differ from antibodies that cause transfusion reactions in that they interact with different epitopes. Warm autoantibodies against other blood group antigens also exist, but they are rare. The underlying causes of haemolytic anaemias usually remain unknown, but some of these conditions are associated with other autoimmune diseases. Anaemia is more often related to the accelerated destruction of sensitised erythrocytes by splenic macrophages than to complement-mediated lysis.

Cold autoantibodies cause erythrocyte lysis as a result of complement fixation. Cold autoantibodies are frequently present in higher titres than warm autoantibodies. They predominantly belong to the IgM class and efficiently bind complement. In most cases, these antibodies are specific for antigens of the Ii blood group system. The I and i epitopes are present on precursor polysaccharide molecules containing ABO system epitopes and result from incomplete glycosylation of the core polysaccharide.

The reaction of cold autoantibodies with erythrocytes occurs in the peripheral circulation—specifically in cutaneous capillaries, where blood Temperature, particularly in winter, may drop below 30 °C. In severe cases, cell aggregation and microvascular thrombosis caused by complement-mediated erythrocyte destruction lead to peripheral tissue necrosis. The severity of the anaemia is therefore directly related to the capacity of the patient's serum to fix complement. (Fc receptor-mediated destruction of sensitised cells in the spleen and liver does not occur in this Setting, as the temperature of these Organs is too high for these antibodies to interact with erythrocytes.)

Cold autoimmune haemolytic anaemias are most frequently observed in elderly individuals. Their cause is unknown, but it should be emphasised that the resulting autoantibodies exhibit a very restricted clonality, indicating the presence of a limited number of autoreactive clones. However, in some cases, the disease is preceded by a Mycoplasma pneumoniae infection; the anaemia then has an acute onset, is short-lived, and is characterised by the presence of polyclonal autoantibodies. Such cases are believed to result from cross-reactivity between bacterial and erythrocyte surface antigens, triggering adverse reactions by bypassing normal tolerance mechanisms (see Chapter 28).

'Drug-induced' reactions to blood components can be caused either by Antibody Binding to substances adsorbed onto cells or by The breakdown of self-tolerance. Drugs (or their catabolic breakdown products) can provoke hypersensitivity reactions against blood cells (including erythrocytes and platelets) via three distinct pathways (Fig. 24.13).

✵ A reaction occurs when a drug binds to blood cells and antibodies are generated against it. This mechanism requires the presence of both the drug and the antibodies. This phenomenon was first described by Ackroyd, who observed thrombocytopenic purpura (platelet destruction leading to a red rash) following the administration of the drug sedormid. Haemolytic anaemias have been observed following the administration of numerous drugs, including penicillin, quinine, and sulphonamides. Such reactions are uncommon.

✵ If 'drug-antibody' immune complexes are adsorbed onto the erythrocyte cell membrane, complement-mediated cell lysis ensues.

✵ When a drug triggers an allergic reaction and autoantibodies are directed against erythrocyte antigens themselves, this also causes damage, as seen, for example, in 0.3% of patients receiving α-methyldopa. The resulting antibodies resemble warm autoantibodies. In such cases, recovery occurs shortly after the drug is withdrawn.

Fig. 24.13. Three mechanisms of drug-induced cellular damage.

1. The drug adsorbs onto cell membranes. Antibodies against the drug bind to The Cell, leading to complement-mediated lysis.

2. Immune complexes consisting of the drug and antibody adsorb onto erythrocytes. This might occur via the Fc receptor, but is more likely mediated by the C3b receptor (CR1). Damage results from complement-mediated cell lysis.

3. The drug, presumably adsorbed onto the cell surface, breaks self-tolerance, possibly by stimulating T-helper (Th) cells. This leads to the production of antibodies against other cell-surface antigens (blood group antigens).

Note that in mechanisms 1 and 2, the presence of the drug on the cell is required for damage to occur, whereas in mechanism 3, cells are destroyed independently of drug adsorption.

Reactions directed against other blood cells in systemic lupus erythematosus and thrombocytopenia

Neutrophil and lymphocyte antibodies. Autoantibodies to neutrophils (Fig. 24.14) are strictly tissue-specific; they bind exclusively to neutrophils and nothing else. (In contrast, antibodies to ABO blood group antigens exhibit virtually no tissue specificity, as these same antigens are present not only on erythrocytes, but also on epithelial Cells of the earlobe, Salivary Glands, and many other tissues.) In systemic lupus erythematosus (SLE), antibodies to both neutrophils and lymphocytes can be detected, but their role in the disease Pathogenesis is relatively minor, likely because these cells very rapidly shed bound antibodies from their surface.

Fig. 24.14. Neutrophil antibodies in SLE, detected by immunofluorescence of normal neutrophils. (Serum from an SLE patient was added to normal neutrophils, followed by a fluorescently labeled antibody to human Fab fragments. The photomicrograph clearly demonstrates the presence of antineutrophil antibodies.) An acute transfusion reaction against neutrophils may be accompanied by a rise in body temperature, presumably due to the release of pyrogens from damaged cells. This indicates that antineutrophil antibodies are capable of damaging neutrophils, although their role in the pathogenesis of SLE remains unclear. (Reproduced by permission from J. Clin. Invest., 1979; 64:902-12.)

Platelet antibodies. Platelet autoantibodies are present in nearly 70% of patients with idiopathic thrombocytopenic purpura (a condition characterized by the accelerated clearance of platelets from the circulation, primarily through phagocytosis by splenic macrophages). Macrophage immune-adherence receptors participate in this platelet elimination.

Thrombocytopenic purpura most commonly develops following bacterial or viral infections, but it can also be associated with autoimmune diseases, including SLE. In SLE, antibodies to platelet membrane cardiolipin are sometimes detected. Autoantibodies to cardiolipin and other Phospholipids can interfere with Blood Coagulation reactions (in which case they are referred to as lupus anticoagulant) and in some cases cause venous thrombosis and recurrent Miscarriage. The mechanisms of drug-induced thrombocytopenia may be similar to those illustrated in Fig. 24.13.