microglobulin

Beta-2 microglobulin (β2M) is a small glycoprotein that forms part of the MHC class I complex on nearly all nucleated cells. It is a nonpolymorphic light chain of the MHC class I molecule and comprises about 12 kDa. The alpha chain of MHC class I noncovalently associates with beta-2 microglobulin in the endoplasmic reticulum, and the complex presents peptide antigens to CD8+ T cells. Beta-2 microglobulin is expressed ubiquitously and is shed into the circulation, where its serum level can be measured.

Serum β2M is filtered by the glomeruli and reabsorbed and degraded in the proximal tubules; thus levels rise in renal impairment. Consequently β2M is used clinically as a marker of renal function and is often elevated in chronic kidney disease and end-stage renal disease. In dialysis patients, accumulation can lead to dialysis-related amyloidosis due to deposition of β2M in bones and joints.

In oncology, serum β2M is often elevated with high tumor burden and has prognostic value in multiple myeloma, some lymphomas, and other hematologic malignancies. It is not specific to a single disease but contributes to risk stratification and monitoring.

Deficiency of β2M due to genetic mutations impairs MHC class I expression and function, producing a form of immunodeficiency known as bare lymphocyte syndrome type I.