Haptoglobin (frequently 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. The haptoglobin-hemoglobin complex will then be removed by the reticuloendothelial system (mostly the spleen). For this reason, haptoglobin levels will be decreased in haemolytic anaemias. Hp also , prevents bacteria from using the iron present in hemoglobin to grow, regulates the activity of many cell types of the immune system, and is an extracellular chaperone. It is produced mostly by hepatocytes but also by other tissues: e.g. skin, lung, and kidney.

Hp in its simplest form cosists 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, therefore are found in humans: Hp1-1, Hp2-1, and Hp2-2. Hp of different phenotypes have been shown to bind hemoglobin with different affinities, with Hp2-2 being the weakest binder.

Hp has been found in all mammals studied so far, some birds e.g. cormorant and ostrich but also, in its simpler form, in bony fish e.g. zebrafish. Interestingly, Hp is absent in at least some amphibians (Xenopus) and neoganthous birds (chicken and goose).

99 Haptoglobin