A humanized IgG1 monoclonal antibody directed against CD11a, the alpha subunit of human leukocyte-function-associated antigen type 1 (LFA-1), with immunosuppressant activity. Efalizumab binds to CD11a, which is expressed on all leukocytes, resulting in a reduction in the cell surface expression of CD11a. In addition, this agent inhibits the binding of LFA-1 to intercellular adhesion molecule-1 (ICAM-1), resulting in the inhibition of leukocyte adherence and the suppression of cell-mediated immunity. LFA-1 binding to ICAM-1 is involved in the activation of T lymphocytes, adhesion of T lymphocytes to endothelial cells, and migration of T lymphocytes to sites of inflammation.