Haptoglobin functions to bind free hemoglobin, released from erythrocytes, which allows degradative enzymes to access the hemoglobin, preventing loss of iron through the kidneys.Haptoglobulin level is used to screen and monitor intravascular hemolytic anemia.

Haptoglobin consists of two alpha and two beta 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. Three phenotypes of Hp are found in humans: Hp1-1, Hp2-1, and Hp2-2. Plasma levels of haptoglobin are highest in individuals with Hp1-1 > Hp2-1 > Hp2-2 individuals. Hp1-1 is the most effective at binding hemoglobin, and Hp2-2 is the least effective. There is a correlation between recurring epileptic seizures and the Hp2-2 phenotype.

Hp 1-1: 86 kDa
Hp 2-1: 200 kDa
Hp 2-2: 400 kDa