β2 microglobulin is also known as Beta-2-microglobulin (B2M), is a component of MHC class I molecules which belongs to the beta-2-microglobulin family. B2M is present on all nucleated cells (excludes red blood cells). B2M 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 of B2M 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. Defects in B2M are the cause of hypercatabolic hypoproteinemia (HYCATHYP).