The CBRM1/5 antibody reacts with an activated form of human CD11b, a 165-170 kD type I transmembrane glycoprotein also known as alphaM integrin, Mac-1, CR3, and C3biR. CD11b non-covalently associates with integrin beta2 (CD18) and is expressed on granulocytes, monocytes/macrophages dendritic cells, NK cells, and subsets of T and B cells. CD11b/CD18 is critical for the transendothelial migration of monocytes and neutrophils, as well as adhesion, phagocytosis and neutrophil activation. CD11b/CD18 interacts with ICAM-1 (CD54), ICAM-2 (CD102), ICAM-4, CD14, CD23, heparin, iC3b, fibrinogen and Factor X. The CBRM1/5 antibody recognizes a subset of CD11b molecules on neutrophils and monocytes activated with chemoattractants or phorbol-esters. This antibody does not recognize "non-activated" CD11b. The epitope recognized by CBRM1/5 is contained in the I domain of the alpha chain. The CBRM1/5 antibody blocks cell adhesion to fibrinogen and ICAM-1.