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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). 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. Levels of beta-2 microglobulin can be elevated in multiple myeloma and lymphoma, though in these cases primary amyloidosis (amyloid light chain) and secondary amyloidosis (amyloid associated protein) are more common.
