An IgM class autoantibody that is activated only when the temperature falls below 100°C. These antibodies may destroy the patient’s red blood cells and are one cause of autoimmune hemolytic anemia. They are found in the serum of patients (esp. those older than 50 years) with atypical (e.g., mycoplasma) pneumonia, infectious mononucleosis, cytomegalovirus infections, mumps, and certain blood diseases. The complement-mediated, autoimmune hemolysis that results is an example of a type II hypersensitivity reaction.