CD11b is an alpha chain integrin that is expressed as heterodimer noncovalently associated with CD18. CD11b is also designated as CR3, Mac-1, Mo-1 in integrin alphaM subunit and has a molecular weight of 165-170 kDa. The protein mediates PMN and monocytes adherence to endothelium and subsequently PMN extravasation to sites of inflammation. CD11b/CD18 interacts with both matrix and cell surface proteins including the complement component iC3b, extracellular matrix proteins and the third extracellular domain of CD54. CD11b is expressed on human NK-cells, monocytes, granulocytes, T-cells, B-cells and dendritic cells. After activation the expression is upregulated. Bear-1 reacts with the CD11b alpha chain of the CD11b/CD18 (p165, 95) integrin heterodimer complex.