Integrin alpha M (ITGAM) is one protein subunit that forms heterodimeric integrin alpha-M beta-2 (α Mβ 2) molecule, also known as (Mac-1) or macrophage-1 antigen (CR3). complement receptor 3 ITGAM is also known as CR3A, and  cluster of differentiation molecule 11B (CD11B). The second chain of α Mβ 2 is the common integrin β 2 subunit known as CD18, and integrin α Mβ 2 thus belongs to the β 2 subfamily (or leukocyte) integrins.
Mβ 2 is expressed on the surface of many leukocytes involved in the innate immune system, including monocytes, granulocytes, macrophages, and natural killer 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 α  Mβ 2 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. 
histopathology, immunohistochemistry with antibodies against CD11B is frequently used to identify macrophages and microglia.
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PDB 1bho EBI.jpg
1bho: MAC-1 I DOMAIN MAGNESIUM COMPLEX
PDB 1bhq EBI.jpg
1bhq: MAC-1 I DOMAIN CADMIUM COMPLEX
PDB 1idn EBI.jpg
1idn: MAC-1 I DOMAIN METAL FREE
PDB 1ido EBI.jpg
1ido: I-DOMAIN FROM INTEGRIN CR3, MG2+ BOUND
PDB 1jlm EBI.jpg
1jlm: I-DOMAIN FROM INTEGRIN CR3, MN2+ BOUND
PDB 1m1u EBI.jpg
1m1u: AN ISOLEUCINE-BASED ALLOSTERIC SWITCH CONTROLS AFFINITY AND SHAPE SHIFTING IN INTEGRIN CD11B A-DOMAIN
PDB 1mf7 EBI.jpg
1mf7: INTEGRIN ALPHA M I DOMAIN
PDB 1n9z EBI.jpg
1n9z: INTEGRIN ALPHA M I DOMAIN MUTANT
PDB 1na5 EBI.jpg
1na5: INTEGRIN ALPHA M I DOMAIN