An IgG class autoantibody that is activated at a temperature of 37°C. These antibodies damage the membranes on the patient’s own red blood cells, which results in their destruction by the spleen, producing an autoimmune hemolytic anemia. The source of the autoantibodies is unclear in 50% of the cases. In the other 50% of patients, they are related to a drug reaction; autoimmune diseases, especially systemic lupus erythematosus; or malignancies.