Beta 2 microglobulin  is a component of  major histocompatibility complex   (MHC) class I molecules, which are present on almost all cells of the body  red blood cells   are a notable exception.  Beta 2 microglobulin   lies lateral to the  alpha 3 chain   on the cell surface.  
 Beta 2 microglobulin  is a  small protein normally found on the surface of many cells, including lymphocytes, and in small amounts in the blood and  urine  .  Testing is done primarily when evaluating a person for certain kinds of  cancer   affecting white blood cells including  chronic lymphocytic leukemia  , non-Hodgkin's lymphoma, and multiple myeloma or  kidney disease .
 beta 2-Microglobulin   (beta 2M), an interesting and underutilized metabolite, can be used in assessing  renal function , particularly in kidney-transplant recipients and in patients suspected of having  renal tubulointerstitial disease  . It also can serve as a nonspecific but relatively sensitive marker of various neoplastic, inflammatory, and infectious conditions.(1)
(1) Annals of Clinical and Laboratory Science , Vol 20, Issue 3, 163-168