© 2018 Elsevier Ltd All rights reserved. Complement receptor 3 (CR3) is a member of the 2 integrin family and was first described in the late 1970s. Integrins are heterodimeric proteins consisting of an α-chain-subunit that is noncovalently associated with one or more α-chains. In addition to CR3, the 2 integrin family contains three other members, LFA-1 (CD11a/CD18), CR4 (CD11c/CD18) and CD11d/CD18. CR3 is primarily expressed on myeloid cells and mediates many different important functions including leucocyte adhesion, activation, recruitment, host defence, phagocytosis and immune tolerance functions through interactions with numerous ligands including iC3b, ICAM-1 and fibrinogen. CR3 expression is controlled by several mechanisms, including at the transcriptional level, through release from intracellular granules following cellular activation with many different mediators, and by cleavage from the cell surface. Analyses of CR3 in different model systems, including CD11b mutant mice, suggest this complement receptor plays key regulatory roles in regulating immune and inflammatory processes.