Autoimmune hemolytic anemias (AIHAs) may occur when specific autoantibodies react with red blood cell (RBC) antigens. Decompensated hemolysis and detectable autoantibodies against RBCs are classical findings. The autoantibodies preferentially react at 37 °C (warm autoantibodies). The majority of these autoantibodies are of the IgG class; IgM and IgA warm autoantibodies are less common. Roughly 50% of the patients have an associated disease (symptomatic or secondary AIHA) – particularly lymphoproliferative and autoimmune disorders in adults, and viral infections in children. The cause of autoimmunization remains obscure in almost all other patients (idiopathic AIHA). Corticosteroids are the mainstay of therapy, but most patients with warm-antibody AIHA require additional treatment with azathioprine; some must receive cyclophosphamide or other drugs. The most effective treatment in cases with life-threatening anemia is the blood transfusion. Cold agglutinins are the cause of hemolysis in about 10% of patients with AIHA. These antibodies exhibit reactivity at temperatures < 37 °C. The most effective treatment is the consistent avoidance of exposure to cold. Donath-Landsteiner antibodies (paroxysmal cold hemoglobinuria) predominantly occur in children with a history of recent viral infection. In these cases, the hemolysis is usually acute and disappears within a few days. Drug-induced hemolysis occurs in about 10% of cases. The causative antibodies react with RBCs either in the presence and absence of the precipitating drug (drug-independent autoantibodies) or only in the presence of the drug and/or its metabolites (drug-dependent antibodies). The former antibodies are of the IgG class and predominantly cause extravascular hemolysis without complement activation, whereas the latter antibodies are of IgG and/or IgM classes that almost invariably cause intravascular hemolysis due to complement activation. Many patients have both types of antibodies. These cases are often confused with AIHA of the warm type.

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