CD11?, also known as integrin alpha-L (ITGAL), is the alpha chain of the lymphocyte function-associated antigen-1 (LFA-1). CD11? combines with the beta chain CD18 (ITGB2) to form LFA-1, an integrin expressed on all leukocytes. LFA-1 plays a central role in leukocyte intercellular adhesion through interactions with its ligands ICAM-1, 2, 3, and 4, F11R/JAM-1, and the secreted form of ubiquitin-like protein ISG15. LFA-1 is involved in a variety of immune phenomena including leukocyte-endothelial cell interaction and transmigration, cytotoxic T cell co-stimulation, cytotoxic NK cell mediated killing, leukocyte clearance, and antibody-dependent killing by granulocytes and monocytes. LFA-1 is required for generation of common lymphoid progenitor cells in bone marrow, indicating a role in lymphopoiesis. High expression of CD11? has been associated with antigen-specific and tumor-reactive T cells.