Warm-type autoantibodies of autoimmune hemolytic anemia (AIHA) are usually IgG but may be IgM or IgA. They are usually Rh specific. Cold-type antibodies are IgM or IgG (Donath-Landsteiner [DL] antibody). IgM antibodies are usually anti-I (occasionally anti-i) and DL antibodies anti-P.
The warm IgG antibodies do not fix complement (C); they cause red blood cell (RBC) destruction predominantly in the spleen as the result of interaction between antibody-coated cells and phagocytes. The cold antibodies are complement fixing; they cause RBC destruction either by intravascular lysis (complement sequence completed) or by interaction between C3-coated RBCs and phagocytes in liver and spleen.
Genetic factors, immunoglobulin deficiency, somatic mutation, viral infections and drugs, and failure of T-lymphocyte function, all probably play a part in breaking immunological tolerance and the development of AIHA.
Dacie JV. Autoimmune Hemolytic Anemia. Arch Intern Med. 1975;135(10):1293–1300. doi:10.1001/archinte.1975.00330100019004