Autoimmune Hemolytic Anemia
- Autoimmune Hemolytic Anemia (AIHA) refers to an acquired hemolytic anemia caused by direct binding of antibodies to erythrocytes, resulting in their destruction. The distinctive laboratory feature of AIHA is a positive direct Coombs Test.
|Etiology and Pathogenesis|
- Taken together, AIHA represents the most common cause of acquired hemolytic anemia; however, a number of distinct insults can trigger generation of these anti-erythrocyte antibodies. Both IgG and IgM auto-antibodies can be generated which are responsible for the two different sub-types of the disease
- Warm AIHA
- Warm AIHA is characterized by the presence of anti-erythrocyte IgG which can bind at "warm" temperatures. When IgG binds the red cell membrane, its Fc Region is recognized by macrophages of the reticuloendothelial system, and the targeted RBC undergoes destruction. This hemolysis is extravascular, largely occurring in the spleen and thus splenomegaly is common.
- Warm AIHAs can occur idiopathically but are often secondary to another process, either a rheumatic disease or frequently lymphomas/leukemias, particularly Chronic Lymphocytic Leukemia
- Cold AIHA
- Cold AIHA is characterized by the presence of anti-erythrocyte IgM which bind most optimally "cold" temperatures, thus often referred to as "Cold Agglutinins". Because binding of IgM to the red cell membrane can directly activate complement, intravascular lysis of the erythrocyte ensues. Cold AIHA typically results during the immune response to certain infections, most characteristically those of Mycoplasma pneumoniae as well as some viruses.
- The most characteristic laboratory consequence of AIHAs is positivity of the direct Coombs Test (see page). However, like all hemolytic anemias AIHAs are characterized by a reticulocytosis and low haptoglobin levels.
- Spherocytes may be visible on peripheral blood smear in patients with AIHA. In this context spherocytes result from partial phagocytosis of the erythrocyte membrane by cells of the reticuloendothelial system.
- AIHAs can result in the classic set of symptoms associated with anemia (see page) as well as an unconjugated hyperbilirubinemia and in some cases jaundice if hemolysis is severe.