Beta2microglobulin

Beta-2-microglobulin is a small protein of about 11.8 kDa that forms the light chain of the major histocompatibility complex (MHC) class I molecule. It non-covalently associates with the heavy chain and peptide that together present endogenous antigens to CD8+ T cells. Beta-2-microglobulin is expressed on the surface of nearly all nucleated cells and is released into the bloodstream and other body fluids.

The protein is encoded by the B2M gene, located on chromosome 15 in humans. It is produced constitutively by many cell types and is shed from cell surfaces into circulation. Because it is normally filtered by the glomeruli and reabsorbed and degraded by the proximal tubules, serum levels reflect both production and renal function.

In clinical practice, beta-2-microglobulin serves as a useful biomarker. Serum levels rise with reduced kidney clearance, making it a marker of glomerular filtration rate and chronic kidney disease. Elevated levels are also associated with higher tumor burden in certain malignancies, notably multiple myeloma and other lymphoid cancers, where it is incorporated into prognostic staging systems. Urinary beta-2-microglobulin can increase with proximal tubule dysfunction, providing a marker of tubular injury. Dialysis-related amyloidosis results from deposition of beta-2-microglobulin-derived amyloid in bone and joints in long-term dialysis patients.

Measurement is typically performed by immunoassays (such as nephelometry or turbidimetry) in serum or plasma, with urine testing used in evaluating tubular function. Overall, beta-2-microglobulin links immune biology to clinical assessment of renal function and certain malignancies in a concise, noninvasive way.