CD11b (integrin ?M subunit) combines with CD18 (integrin ?2 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. 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.