The M18/2 antibody specifically recognizes the common β2 chain of LFA-1 (CD11a/CD18, αLβ2 integrin), Mac-1 (CD11b/CD18, αMβ2 integrin), and gp150, 95 (CD11c/CD18, αXβ2 integrin). Expression of CD18 is limited to leukocytes, where it is widely distributed in consort with the three integrin α chains (CD11a, CD11b, and CD11c). Among splenocytes, NK cells have the highest density of CD18, and T lymphocytes express a higher density than the remaining cells. The β2 integrins are important mediators of leukocyte-endothelium interactions. It has been reported that M18/2 antibody blocks in vivo metastasis of the LB lymphoma to the spleen and that it blocks in vitro formation of aggregates of LB cells and splenocytes. However, other reports indicate that mAb M18/2 has no effect on CTL-mediated killing, adherence of C3bi-sensitized erythrocytes to Mac-1, antigen-specific binding of T cells to antigen-producing cells, or rejection of cardiac allografts. Recent in vitro studies indicate that M18/2 antibody stimulates adhesion of Mac-1 to its ligands C3bi and ICAM-1, and it stimulates adhesion of LFA-1 to ICAM-1, but it has no effect upon the interactions of LFA-1 with ICAM-2 nor ICAM-3.