Cr3

CR3, or complement receptor type 3, is a leukocyte integrin that plays a central role in the innate immune response. It is expressed on neutrophils, monocytes, macrophages, and some dendritic cells, and consists of a noncovalent heterodimer of the αM subunit (CD11b) and the β2 subunit (CD18), commonly referred to as MAC-1. The CR3 complex is encoded by the ITGAM and ITGB2 genes.

CR3 binds a variety of ligands, most notably iC3b, a fragment of the complement component C3. This interaction enables phagocytosis of complement-opsonized targets. It also participates in cell adhesion by interacting with ligands such as ICAM-1 on endothelial cells, contributing to the leukocyte adhesion cascade and migration into tissues. Additional ligands include fibrinogen and other extracellular matrix components, supporting roles in inflammation and tissue remodeling.

Activation of CR3 involves inside-out signaling that increases its affinity for ligands and promotes cytoskeletal rearrangements necessary for phagocytosis and motility. In addition to pathogen clearance, CR3 participates in the clearance of apoptotic cells and presents antigens to influence adaptive immune responses. Its signaling can modulate cytokine production and reactive oxygen species generation in phagocytes.

Clinical and research relevance: genetic variation in ITGAM (CD11b) has been associated with susceptibility to autoimmune diseases such as systemic lupus erythematosus, possibly by altering CR3 function. CR3 can influence outcomes in infections and inflammatory diseases, and it is a target of interest for therapies aiming to modulate inflammatory responses or enhance host defense.

See also: complement system; integrins; CD11b; CD18; MAC-1.