The CD11b (HI11b) antibody recognizes the 165-kDa integrin αM-chain associated with CD18, the 95-kDa integrinβ2, which form a heterodimer glycoprotein called CD11b/CD18 (Mac-1). Leukocytes express four CD11/CD18 integrins which include a common β2 subunit and distinctive α chains. Besides CD11b/CD18, the other three integrins are known as CD11a/CD18, CD11c/CD18 and CD11d/CD18. Expression of the CD11b subunit on the leukocyte cell surface requires the presence of the CD18 subunit. Patients who have mutated CD18 genes do not express cell membrane Mac-1 (or the other β2 integrins), a condition known as leukocyte adhesion deficiency 1 (LAD-1), in which patients have major problems with bacterial infections. Mac-1 is highly expressed on neutrophils and monocytes/macrophages, a subset of NK cells, and activated lymphocytes. Mac-1 operates as a phagocytic receptor, has a role in myeloid cell transmigration and is also a receptor for iC3b, CD54 (ICAM-1), CD102 (ICAM-2) and fibrinogen.