Immune hemolytic anemia occurs when antibodies form against the body's own red blood cells. The antibodies may be acquired by blood transfusion, pregnancy (if the baby's blood type is different from the mother's), as a complication of another disease, or from a reaction to medications.

If the cause of antibody formation is disease or medication, it is referred to as secondary immune hemolytic anemia. The antibodies will destroy the blood cells because they are recognized as foreign substances within the body.

The cause may also be unknown, as in idiopathic autoimmune hemolytic anemia, which accounts for one-half of all immune hemolytic anemias. The onset of the disease may be quite rapid and very serious, or it may remain mild and not require specific therapy. Risk factors are related to the causes.