TYPE II HYPERSENSITIVITY
Type II or cytotoxic hypersensitivity refers to those situations in which antibody activates complement causing tissue damage. Examples include transfusion reactions to mismatched blood, and hemolytic disease of the newborn (HDN). In addition, autoimmune reactions that involve lysis of cells can be included here. Autoimmune diseases have already been discussed above and transfusion and HDN are described, hence only a brief discussion of the causes and consequences of the transfusion of ABO incompatible blood will be given here.
The ABO blood group system contains four blood groups: A, B, AB, or O, according to the types of antigens found on erythrocyte membranes. In the plasma, there are also antibodies to the blood group antigens thatare not present on the erythrocyte membranes. Thus, individuals with blood group A have anti-B antibodies in their plasma, while those of blood group B have anti-A (Figure 5.13). These antibodies are known as isohemagglutinins, and usually belong to the IgM class, which are efficient activators of complement. If a blood group A individual is transfused with group B blood, then antibodies from the donated blood will bind to recipient erythrocytes, activate complement and cause their lysis. Similarly, antibodies in the recipient will lyse the donated erythrocytes. The sudden and simultaneous lysis of cells leads to kidney failure and death.