Beta-2 microglobulin

β₂ microglobulin also known as B2M is a component of MHC class I molecules, which are present on all nucleated cells (excludes red blood cells). In humans, the β₂ microglobulin protein is encoded by the B2M gene.

Structure and function

β₂ microglobulin lies beside the α₃ chain on the cell surface. Unlike α₃, β₂ has no transmembrane region. Directly above β₂ (that is, further away from the cell) lies the α₁ chain, which itself is next to the α₂.

β₂ microglobulin associates not only with the alpha chain of MHC class I molecules, but also with class I-like molecules such as CD1 and Qa.

An additional function is association with the HFE protein, together regulating the expression of hepcidin in the liver which targets the iron transporter ferroportin on the cytoplasmic membrane of enterocytes and macrophages for degradation resulting in decreased iron uptake from food and iron release from recycled red blood cells respectively. Loss of this function causes iron excess and hemochromatosis.