β2 microglobulin also known as B2M is a component of MHC class I molecules, which are present on almost all cells of the body. β-2-Microglobulin associates with the α heavy chain of MHC and forms an immunoglobulin domain-like structure that mediates proper folding and expression of MHC class 1 molecules. Defects in B2M are the cause of hypercatabolic hypoproteinemia. Affected individuals show marked reduction in serum concentrations of immunoglobulin and albumin, probably due to rapid degradation.