CD11

CD11 refers to the alpha chain subunits of the beta-2 integrin family expressed on leukocytes. The known CD11 subunits are CD11a (ITGAL), CD11b (ITGAM), CD11c (ITGAX), and CD11d (ITGAD). Each pairs with the beta-2 integrin CD18 to form a heterodimeric receptor: LFA-1 (CD11a/CD18), Mac-1 (CD11b/CD18), CR4 (CD11c/CD18), and αDβ2 (CD11d/CD18).

These receptors mediate adhesion of leukocytes to the endothelium, migration into tissues, and various effector functions such as phagocytosis and antigen presentation. LFA-1 and Mac-1 participate in firm adhesion and diapedesis by binding ligands such as ICAM-1 and ICAM-2; Mac-1 also recognizes iC3b among other ligands; CR4 and αDβ2 contribute to adhesion and phagocytosis in myeloid cells.

Expression patterns vary among subtypes: CD11a is broadly expressed on most leukocytes; CD11b is prominent on neutrophils and monocytes; CD11c is characteristic of dendritic cells and some monocytes; CD11d is found on tissue macrophages in certain contexts. The CD11 family thus serves as a functional set of adhesion receptors that regulate leukocyte trafficking and activation during immune responses.

Clinical and research relevance includes the role of β2 integrin defects in immunodeficiency disorders such as leukocyte adhesion deficiency, where impaired adhesion and migration lead to recurrent infections. Abnormal CD11 function is also linked to inflammatory diseases and cancer. In research and diagnostic settings, CD11 molecules are commonly used as surface markers to identify and characterize leukocyte subsets by flow cytometry.