CD11b, also known as Integrin alpha M and Macrophage-1 Antigen (MAC-1) alpha subunit, is an Integrin alpha chain family member that binds with the Integrin beta 2 subunit, also called CD18, to form the non-covalent heterodimer Integrin alpha M/beta 2, also known as MAC-1 and Complement Receptor Type 3 (CR3). Integrin alpha M/beta 2 is expressed on the surface of many leukocytes including granulocytes, macrophages, dendritic cells, and natural killer cells. It has also been shown to be expressed on microglia, the resident immune cells in the brain. Upon activation, Integrin alpha M/beta 2 binds to several ligands, including ICAM-1, Fibrinogen, and the C3 complement fragment C3bi, to mediate phagocyte adhesion, migration and ingestion of complement-opsonized particles. Human CD11b/Integrin alpha M is 1152 amino acids (aa) in length with a predicted molecular weight of approximately 127 kDa. It contains a N-terminal von Willebrand factor, the type A domain (aa 150-328) and seven FG-GAP repeats, which are thought to be responsible for folding CD11b/Integrin alpha M into a beta propeller structure. Human CD11b/Integrin alpha M shares 74% aa sequence identity with the mouse and rat orthologs.