Haptoglobin (Hp)

Haptoglobin (abbreviated as Hp) is a protein in the blood plasma that binds free hemoglobin released from erythrocytes with high affinity and thereby inhibits its oxidative activity. Hp in its simplest form consists of two α- and two β-chains, connected by disulfide bridges. The chains originate from a common precursor protein which is proteolytically cleaved during protein synthesis. Hp exists in two allelic forms in the human population, so called Hp1 and Hp2; the latter one having arisen due to the partial duplication of Hp1 gene. Three phenotypes of Hp are found in humans: Hp1–1, Hp2–1, and Hp2–2. Hp phenotypes are associated with pathogenesis of a number of human disorders, such as diabetes, cardiovascular disease, etc. Hp plays a role in the host defence responses to infection and inflammation, acting as a natural antagonist for receptor-ligand activation of the immune system, also.