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.
