Integrin alpha M (ITGAM) is one protein subunit that forms heterodimeric integrin alpha-M beta-2 (αMβ2) molecule, also known as macrophage-1 antigen (Mac-1) or complement receptor 3 (CR3). ITGAM is also known as CR3A, and cluster of differentiation molecule 11B (CD11B). The second chain of αMβ2 is the common integrin β2 subunit known as CD18, and integrin αMβ2 thus belongs to the β2 subfamily (or leukocyte) integrins. αMβ2 is expressed on the surface of many leukocytes involved in the innate immune system, including monocytes, granulocytes, macrophages, and natural killer cells and subsets of T and B cells. It mediates inflammation by regulating leukocyte adhesion and migration and has been implicated in several immune processes such as phagocytosis, cell-mediated cytotoxicity, chemotaxis and cellular activation. It is involved in the complement system due to its capacity to bind inactivated complement component 3b (iC3b). The ITGAM (alpha) subunit of integrin αMβ2 is directly involved in causing the adhesion and spreading of cells but cannot mediate cellular migration without the presence of the β2 (CD18) subunit. In genomewide association studies, single nucleotide polymorphisms in ITGAM had the strongest association with systemic lupus erythematosus, with an odds ratio of 1.65 for the T allele of rs9888739 and lupus. In histopathology, immunohistochemistry with antibodies against CD11B is frequently used to identify macrophages and microglia. Function of CD11b CD11b, as an integrin molecule on the surface of leukocytes, plays an important role in cell migration, adhesion, and transmigration across blood vessels, because it can bind to components of extracellular matrix and intracellular adhesion molecules (ICAMs) on the endothelial surface. This process is important for leukocyte recruitment into the site of inflammation. Moreover, there are other important processes with CD11b involvement, more precisely Mac-1 integrin involvement as a whole. One of which is phagocytosis of opsonised particles by a complement component iC3b. Such opsonised particles could be bacteria, apoptotic cells, and even immune complexes. CD11b binding to iC3b leads to a production of anti-inflammatory cytokines, e.g., interleukin 10 (IL-10) and tumour growth factor beta (TGFβ). This process is important for regulation of the inflammatory milieu. CD11b is also involved in the differentiation of osteoclasts, bone remodelling cells. Mac-1 is expressed in osteoclast progenitors, and it seems that it is a part of a negative feedback of osteoclastogenesis. CD11b also modulates other functions of leukocytes, e.g. oxidative burst, apoptosis, binding of fibrinogen etc. On circulating leukocytes, CD11b is expressed in