B2M

Beta-2-microglobulin (B2M) is a small, 12-kilodalton protein encoded by the B2M gene in humans. It is a light chain component of major histocompatibility complex class I (MHC I) molecules and is expressed on the surface of nearly all nucleated cells. B2M is shed from the cell surface and circulates in serum, urine, and other body fluids.

Biologically, B2M pairs noncovalently with the MHC class I heavy chain and is required for proper folding and stable expression of MHC I at the cell surface. This complex presents endogenous peptides to CD8+ T cells, playing a central role in immune surveillance. B2M itself is not typically presented as an antigen; its levels reflect turnover of MHC I molecules and renal clearance.

Clinically, serum B2M is used as a biomarker of renal function because it is cleared by the kidneys. Levels rise in renal failure and accumulate in patients undergoing long-term hemodialysis. Elevated B2M can also be observed in inflammatory states, infections, autoimmune diseases, and various malignancies, including lymphomas and multiple myeloma, where higher levels correlate with tumor burden and poorer prognosis.

In dialysis-related amyloidosis, beta-2-microglobulin deposits in joints and tissues, leading to pain and mobility problems in long-term dialysis patients. B2M concentrations are measured by immunoassays such as nephelometry, turbidimetry, or ELISA; reference ranges vary by assay, but typical healthy adults have about 1–2.5 mg/L in serum. Values higher than this are considered elevated and interpreted in the context of renal function and disease.

Note: In other contexts, B2M may refer to entities other than beta-2-microglobulin; this article focuses on the protein and its roles in immunology and medicine.