Complement receptor 3 (CR3)(CD11b/CD18) is a human cell surface receptor found on polymorphonuclear leukocytes (mostly neutrophils), NK cells, and mononuclear phagocytes like macrophages. CR3 is a pattern recognition receptor, capable of recognizing and binding to many molecules found on the surfaces of invading bacteria. CR3 also recognizes iC3b when bound to the surface of foreign cells. Binding to the receptor causes phagocytosis and destruction of the foreign cell.
CR3 belongs to a family of cell surface receptors known as integrins (because they share this particular β chain, they are referred to as β2-integrins), which are extremely widely distributed throughout nature and which generally are important in cellular adhesion and cell-cell interactions in a variety of cells and circumstances.
Upregulation of Mac-1 in the presence of certain factors such as IL-2 may cause a prolongation of the life of the immune cell while the presence of TNF-α induces apoptosis and selective removal of the cell.
A fully activated neutrophil may express on its membrane 200,000 or more CR3 molecules.
Absence of CR3 results in reduced binding and ingestion of Mycobacterium tuberculosis in mice. In human mononuclear phagocytes, phagocytosis of Mycobacterium tuberculosis is mediated in part by human monocyte complement receptors including CR3.
CR3 has also been shown to mediate phagocytosis of the Lyme disease causing bacterium, Borrelia burgdorferi, in the absence of iC3b opsonization. 
^Schlesinger LS, Bellinger-Kawahara CG, Payne NR, Horwitz MA. (1990). "Phagocytosis of Mycobacterium tuberculosis is mediated by human monocyte complement receptors and complement component C3". J. Immunol144 (7): 2771–80. PMID2108212.
^Hawley KL, Olson Jr. CM. (2012). "CD14 Cooperates with Complement Receptor 3 to mediate MyD88-Independent Phagocytosis of Borrelia burgdorferi". Proc. Natl Acad of Sci USA109 (4): 1222–32. doi:10.1073/pnas.1112078109. PMID22232682.
Rooyakkers AW, Stokes RW (September 2005). "Absence of complement receptor 3 results in reduced binding and ingestion of Mycobacterium tuberculosis but has no significant effect on the induction of reactive oxygen and nitrogen intermediates or on the survival of the bacteria in resident and interferon-gamma activated macrophages". Microb. Pathog.39 (3): 57–67. doi:10.1016/j.micpath.2005.05.001. PMID16084683.