The beta-2 integrins, a family of four cell surface transmembrane glycoproteins expressed only on leukocytes, include CD11a/CD18, CD11b/CD18, CD11c/CD18, and CD11d/CD18. They consist of a common beta subunit (CD18) and homologous alpha subunits (CD11a-d) that bind noncovalently to form an alpha/beta heterodimer.
Microglia are the resident macrophages of the central nervous system, and the first line of immune defense. Pioneering antibody research in the 1990's identified the Integrin beta 2 protein (also called ITGB2, complement receptor 3, CR3, CD18, and Mac-1) as one of the key proteins in the recruitment of microglial cells. The ITGB2 subunit A is commonly called CD11b, and antibodies to this subunit are widely used as microglial markers.
CD11b is an integrin family member which pairs with CD18 to form the CR3 heterodimer. CD11b is expressed on the surface of many leukocytes including monocytes, neutrophils, natural killer cells, granulocytes and macrophages, as well as on 8% of spleen cells and 44% of bone marrow cells. Functionally, CD11b regulates leukocyte adhesion and migration to mediate the inflammatory response.