β2-microglobulin is a low molecular weight protein that was first isolated from urine of patient with tubular renal diseases and workers exposed to cadmium. β2M is found on the surface of nearly all nucleated cells, where it constitutes the light chain of class 1 human leukocytes antigens (HLA) .As a result of metabolism and degradation of HLA , β2M appears in its free form and can be found at low concentrations in serum, urine and other body fluids.
Free β2M is eliminated from the body by glomerular filtration followed by tubular reabsorption and degradation. Serum levels of β2M are frequently elevated in patients with a variety of lymphoproliferative and inflammatory disorders reflecting an augmentation of synthesis.
Abnormally high level of β2M are also associated with renal dysfunction and reduced glomerular filtration reflecting a reduced urinary excretion. Acute rejection of renal transplant patients elevates serum β2M days before other markers like creatinine. In some renal disorders, β2M is also determined in urine being its concentration abnormally high in aminoglycoside or lithium toxicity, heavy metal poisoning and acute tubular necrosis. It also helps to differentiate infections of the upper urinary tract from those of the lower urinary tract.