<p>Beta-2-microglobulin constitutes the 12 kDa light chain of the class I major histocompatibility complex (MHC-I) on the surface of many cells, to which it is non-covalently associated. It is also found in the serum in a free form. Beta-2-microglobulin co-association with the MHC class I heavy chain is prerequisite for functional expression of the protein complex on the cell surface. Together these function in the presentation of nine amino acid peptides to the T-cell receptor of CD8-bearing T lymphocytes and killer inhibitory receptors on natural killer cells.</p>