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Immunohemolytic Anemias.

caused by antibodies that react against normal or altered RBC membranes.
Immunohemolytic anemias are classified on the basis of the nature of the antibody involved:

Warm Antibody Immunohemolytic Anemias :

These are characterized by the presence of immunoglobulin G (IgG) antibodies usually against Rh antigen which are active at 37°C.

Cold Antibody Immunohemolytic Anemias :

These anemias are characterized by the presence of low-affinity immunoglobulin M (IgM) antibodies, which bind to RBC membranes at temperatures 0-4°C, as may be encountered in distal body parts (e.g., hands, toes). Fixaton of C3b can cause intravascular Hemolysis. But C3b is active at 37°C. When such antibody- and complement-coated cells travel to warmer areas, the weakly bound IgM antibody is released, and the cell is left with a coating of C3b. So RBC coated with C3b is phagocytosed - extravascular hemolysis .