Haptoglobin phenotypes in health and disorders.

Haptoglobin is a positive acute phase protein that binds free hemoglobin and removes it from the circulation to prevent kidney injury, and iron lossfollowing hemolysis. Also, by bindingfree hemoglobin, haptoglobin functions as an antioxidant. In addition, haptoglobin acts as a potent immunosuppressor of lymphocyte function and modulates the helper T-cell type 1 and type 2 (Th1/Th2) balance within the body. Three major haptoglobin phenotypes are known to exist (Hp 1-1, Hp 2-1, and Hp 2-2). Hp 1-1 is biologically the most effective in binding free hemoglobin and suppressing inflammatory responses associated with free hemoglobin. Hp 2-2 is biologically the least active, and Hp 2-1 is moderately active. The possible association of allelic polymorphism of haptoglobin with various pathologic conditions such as coronary artery disease has been studied. This article reviews the known functions of haptoglobin and the present understanding of a possible association of haptoglobin phenotypes with pathogenesis of a number of human disorders.