Beta 2 microglobulin (B2MG) is initially discovered as a small serum protein of 12,000 Dalton. The small size allows B2MG to pass through the glomerular membrane, but it is normally completely reabsorbed in the proximal tubules. B2MG normally presents in minute amount, but increases its expressions, or adapts to the fibrillary configuration in certain pathological states. B2MG is found to be a subunit of class I major histocompatibility complex (MHC I) that is expressed on all nucleated cell excluding red blood cells. B2MG non-covalently links with the a3 domain of human leukocyte antigen (HLA) to form MHC I. Inside cytoplasm, MHC I binds to intracellular degraded foreign peptides, or self-peptides in some conditions, through the a1 and a2 variable domains of HLA. Subsequently, MHC I is anchored to the cell surface through the a3 domain of HLA. By way of this procedure, MHC I presents the engulfed foreign peptides to (CD8+) cytotoxic T-cell, thus, plays an important role in cell-mediated immunity. When binding with HFE protein, B2MG was found to increase iron uptake. B2MG is also associated with MHC I - like molecules such as CD1 and Qa. This immunoassay is a tool for investigation of the function and regulation of the protein.