| 背景 | beta2-Microglobulin (b2M) is a 12 kDa secreted polypeptide that serves as the light chain of Class I MHC molecules. Possessing an Ig-like domain, b2M noncovalently associates with both 44 kDa classical (HLA-A, -B, -C) and 40 kDa non-classical (HLA-E, -F, -G) Class I MHC heavy chains as well as with 43-49 kDa Class I non-MHC heavy chains (CD1). b2M is expressed on nearly all nucleated cells, with neurons being a notable exception. Circulating b2M is generated during normal HLA turnover. It can also dissociate from the MHC complex and circulate as full length and N-terminal truncated peptides of 93, 91, and 90 amino acids. It has been measured in a variety of body fluids, including serum, plasma, saliva, CSF, and urine. b2M freely passes through the glomerular membrane, but it is 99% actively reabsorbed and degraded in the proximal tubule cells. Circulating b2M levels are elevated in rheumatoid arthritis, systemic lupus erythematosus, viral infections, and conditions with decreased glomerular filtration. Human b2M shares 70% and 75% amino acid sequence identity with mouse and rat b2M, respectively. |
