beta-2-microglobulin

β2 microglobulin is a component of MHC class I molecules, which are present on almost all cells of the body (red blood cells are a notable exception). β2 microglobulin lies lateral to the α3 chain on the cell surface. Unlike α3, β2 has no transmembrane region. Directly above β2 (i.e. away from the cell) lies the α1 chain, which itself is lateral to the α2. β2 microglobulin associates not only with the alpha chain of MHC class I molecules, but also with class I-like molecules such as CD1 and Qa.
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[MY-kroh-GLOB-yoo-lin]
A small protein normally found on the surface of many cells, including lymphocytes, and in small amounts in the blood and urine. An increased amount in the blood or urine may be a sign of certain diseases, including some types of cancer, such as multiple myeloma or lymphoma.