The antibodies seen in warm-antibody autoimmune hemolytic anemias react at 37degC in vitro, are composed of IgG, and do not fix complement. Instead, immunoglobulin-coated red blood cells are removed by splenic macrophages that recognize the Fc portion of the immunoglobulin. These warm IgG antibodies are found in patients with malignant tumors, especially leukemia-lymphoma; they are associated with the use of such drugs as C-methyldopa and they are also found in the autoimmune diseases, especially lupus erythematosus.> Cold-antibody autoimmune hemolytic anemia (cold AIHA) is subdivided into two clinical categories based on the type of anti- bodies involved. These two types of cold antibodies are cold agglutinins and cold hemolysins.> Cold agglutinins are monoclonal IgM antibodies that react at 4 to 6degC. They are called agglutinins because the IgM can agglutinate red cells due to its large size (pentamer)> Additionally, IgM can activate complement, which may result in IV hemolysis. Mycoplasma pneumonitis and infectious mononucleosis are classically associated with cold-agglutinin formation. In contrast, cold hemolysins are seen in patients with paroxysmal cold hemoglobinuria (PCH)> These cold hemolysins are unique because they are biphasic antierythrocytic autoantibodies. These antibodies arc IgG that is directed against the P blood group antigen. They are called biphasic because they attach to red cells and bind complement at low temperatures, but the activation of complement does not occur until the temperature is increased.> This antibody, called the Donath-Landsteiner antibody was previously associated with syphilis, but may follow various infections, such as mycoplasmal pneumonia.">