Autoimmune Hemolytic Anemia (AIHA) Except for countries where malaria is endemic, AIHA is the most common form of acquired hemolytic anemia. In fact, not quite appropriately, the two phrases are sometimes used synonymously. Pathophysiology AIHA is caused by an autoantibody directed against a red cell antigen, ., a molecule present on the surface of red cells. The autoantibody binds to the red cells. Once a red cell is coated by antibody, it will be destroyed by one or more mechanisms. In most cases the Fc portion of the antibody will be recognized by the Fc receptor of macrophages, and this. | Chapter 101. Hemolytic Anemias and Anemia Due to Acute Blood Loss Part 14 Autoimmune Hemolytic Anemia AIHA Except for countries where malaria is endemic AIHA is the most common form of acquired hemolytic anemia. In fact not quite appropriately the two phrases are sometimes used synonymously. Pathophysiology AIHA is caused by an autoantibody directed against a red cell antigen . a molecule present on the surface of red cells. The autoantibody binds to the red cells. Once a red cell is coated by antibody it will be destroyed by one or more mechanisms. In most cases the Fc portion of the antibody will be recognized by the Fc receptor of macrophages and this will trigger erythrophagocytosis Fig. 101-7 . Thus destruction of red cells will take place wherever macrophages are abundant . in the spleen liver and bone marrow. Because of the special anatomy of the spleen this organ is particularly efficient in trapping antibody-coated red cells and often this is the predominant site of red cell destruction. Although in severe cases even circulating monocytes can take part in this process most of the phagocytosis-mediated red cell destruction takes place in the spleen and liver and it is therefore called extravascular hemolysis. In some cases the nature of the antibody is such usually an IgM antibody that the antigen-antibody complex on the surface of red cells is able to activate complement C . As a result a large amount of membrane attack complex will form and the red cells may be destroyed directly known as intravascular hemolysis. Figure 101-7 Mechanism of antibody-mediated immune destruction of red cells. From N Young et al Clinical Hematology. Copyright Elsevier 2006 with permission. Clinical Features The onset of AIHA is very often abrupt and can be dramatic. The hemoglobin level can drop within days to as low as 4 g dL the massive red cell removal will produce jaundice and often the spleen will be enlarged. When this triad is present the suspicion of AIHA must be
