1. cold-antibody type, caused by hemagglutinating antibody (usually IgM class) maximally active at 4°C; and resulting from severe hemolysis in cold hemagglutinin disease;
2. warm-antibody type, acquired hemolytic anemia due to serum autoantibodies (usually IgG class) maximally active at 37°C; that react with the patient’s red blood cells; it varies in severity, occurs in all age groups of both sexes, and may be idiopathic or secondary to neoplastic, autoimmune, or other disease. The Coombs test is positive for IgG and complement, IgG alone, or complement alone.