CD11b, the integrin aM subunit, combines with CD18, the integrin ß2 subunit, to form the integrin Mac-1, also known as complement receptor 3 (CR3), which mediates adhesion to C3bi and ICAM-1/CD54. Mac-1 is expressed at varying levels on granulocytes, macrophages, dendritic cells, NK cells, and B-1 cells in the peritoneal and pleural cavities. Mac-1 expression is rapidly up-regulated on neutrophils after activation and in the same time period that CD62L is shed from the cell surface. In addition to its role in binding C3bi on opsonized targets and mediation of the subsequent ingestion process, Mac-1 is important as an adhesion molecule in the transendothelial migration of monocytes and neutrophils. Cross-reaction of M1/70 to human CD11b has been observed on monocytes, polymorphonuclear leukocytes, and NK cells.