Warm Autoimmune Hemolytic Anemia

Warm autoimmune hemolytic anemia (warm AIHA) occurs when IgG autoantibodies bind antigens on the patient’s red blood cells at normal body temperature. The coated cells are recognized by macrophages, chiefly in the spleen, and removed from circulation. This is predominantly extravascular hemolysis: red cells are destroyed in the reticuloendothelial system rather than bursting directly in the bloodstream. The marrow responds by increasing red-cell production. If it can keep pace, the reticulocyte count rises. If destruction outstrips production, hemoglobin falls and oxygen delivery declines. Fatigue, exertional dyspnea, palpitations, dizziness, and tachycardia follow because the heart is trying to deliver adequate oxygen with fewer functioning red cells. Warm AIHA is the most common autoimmune hemolytic anemia subtype. About half of cases are primary, but a new diagnosis should always prompt investigation for a secondary cause. Associated conditions include chronic lymphocytic leukemia, non-Hodgkin lymphoma, systemic lupus erythematosus, immunodeficiency, infection, transplantation, and some medications, including immune checkpoint inhibitors.