Autoimmune Hemolytic Anemia
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Autoimmune hemolytic anemia (AIHA) results from autoantibodies targeted to antigens on the patient’s RBCs, resulting in either extravascular hemolysis (removal of RBC by tissue macrophages in the liver or spleen) or complement-mediated intravascular hemolysis.
There are two main types of AIHA: warm and cold AIHA. Warm AIHA antibodies interact best with RBCs at 37°C, whereas cold antibodies (or cold agglutinins) are most active at temperatures below 37°C and almost always fix complement.
- Warm AIHA is usually caused by an IgG autoantibody. It may be idiopathic or secondary to an underlying process (i.e., lymphoma, chronic lymphocytic leukemia [CLL], collagen vascular disorder, or drugs [see Table 21-5]).
- Cold AIHA (or cold agglutinin disease [CAD]) is typically caused by an IgM autoantibody.
- The acute form of CAD is often secondary to an infection (Mycoplasma, Epstein–Barr virus).
- The chronic form is due to a paraprotein associated with lymphoma, CLL, or Waldenström macroglobulinemia (WM) in approximately one-half of cases and is primary (idiopathic) in the others.