Type IIA Autoimmune reaction:
Autoimmune Hemolytic Anemia (AIHA)
AIHA is an example of type IIA autoimmunity. In this disorder, a self-antigen on the surface of erythrocytes elicits an autoantibody response, resulting in the binding of autoantibody to the erythrocytes surface followed by destruction of the antibody-coated erythrocytes by the reticuloendothelial system of the spleen and liver. The mechanism of hemolysis depends on the type of autoantibodies. Autoimmune hemolysis is classified into two groups on the basis of thermal reactivity of the autoantibodies. Warm autoantibodies react optimaly at temperatures of 35-40 C, whereas cold aglutinins and other cold-reactive autoantibodies react maximally at 4C. Warm autoantibodies are typically polyclonal IgG but may also be IgM or IgA. Most are Ig1 subclass antibodies reactive with Rh antigens. These antibodies are detected by the direct antiglobulin (Coombs) test. Erythrophagocytosis mediated by Fc receptors on Kupffer cells in the liver and macrophages in the splenic marginal zone is generally the major mechanism of erythrocyte destruction in patients with warm autoantibodies.