CD11bCD18

CD11bCD18 is the integrin αMβ2, a heterodimer composed of the CD11b (ITGAM) and CD18 (ITGB2) subunits. Also known as Mac-1 or complement receptor 3 (CR3), it belongs to the β2 integrin family and participates in leukocyte adhesion, migration, and phagocytosis. The extracellular αM I-domain contains the primary ligand-binding site, including the metal ion-dependent adhesion site (MIDAS), which coordinates divalent cations to enable binding to various ligands.

Expression of CD11bCD18 is primarily on neutrophils, monocytes, macrophages, and certain dendritic cells, with additional expression on some natural killer cells and eosinophils. Its activation state is regulated by intracellular signaling: upon chemokine or other receptor stimulation, the integrin undergoes conformational changes from a low-affinity, bent form to a high-affinity, extended form, promoting firm adhesion to endothelium and subsequent transmigration (diapedesis) to sites of inflammation.

Ligands include intercellular adhesion molecule-1 and -2 (ICAM-1/ICAM-2) on activated endothelium, as well as iC3b, a cleavage product of C3 that marks opsonized particles for phagocytosis. Additional interactions with ligands such as fibrinogen and other extracellular matrix components support adhesion, phagocytosis, and clearance of opsonized targets.

Clinical and research relevance centers on its role in host defense and inflammatory diseases. ITGB2 (CD18) mutations cause leukocyte adhesion deficiency type I, reflecting the essential function of β2 integrins in leukocyte trafficking. Mac-1 activity is a potential target in inflammatory disorders and tissue injury, where modulation can influence leukocyte recruitment and phagocytic responses.