β2-microglobulin is a small molecule globulin produced by lymphocytes, platelets, and polymorphonuclear leukocytes. It is a single-chain polypeptide consisting of 99 amino acids. It is the β-chain (light chain) portion of human leukocyte antigen (HLA) on the cell surface, and is similar in structure to the immunoglobulin stable region. Normal human β2-microglobulin synthesis rate and release from the cell membrane are quite constant. β2-microglobulin can be freely filtered from the glomerulus, 99.9% is absorbed in the proximal renal tubules, and in renal tubular epithelial cells decomposition and destruction; therefore, β2-microglobulin is normally excreted in a very small amount. The measurement of blood β2 microglobulin helps to reflect the impairment of glomerular filtration function, whether the filtration load is increased, and to monitor the function of proximal tubules. The measurement of urine β2 microglobulin helps to identify upper and lower urinary tract infections. Upper urinary tract infections can easily affect the reabsorption of molecular proteins by the renal tubules, and urine β2 microglobulin is elevated, while lower urinary tract infections do not increase.