β₂ microglobulin also known as B2M is a component of MHC class I molecules, which are present on all nucleated cells (but not red blood cells). β₂ microglobulin lies lateral to the α₃ chain on the cell surface. Unlike α₃, β₂ has no transmembrane region. Directly above β₂ lies the α₁ chain, which itself is lateral 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.

The complete amino acid sequence of the beta-2-microglobulin protein. It is a single polypeptide chain with a molecular mass of 11.6 kD that shows homology to immunoglobulins and HLA, suggesting a common evolutionary origin.