The interaction of T lymphocytes with antigen-presenting cells (APCs) is one of the initial steps in T lymphocyte activation of an immunologic response to what is perceived by the immune system to be a foreign antigen. Although much attention has been focused on the primary interaction of the T lymphocyte receptor with the major histocompatability complex (MHC)-antigen complex on the APC, several other cell surface components are also involved in, and necessary for, T lymphocyte activation. Ligand pairs necessary for T lymphocyte activation, located on the cell surface of the T lymphocytes and the APC, respectively, include CD2/LFA-3, LFA-1/ICAM-1 (also ICAM-2 and ICAM-3), CD281B7, CD4/MHC Class II, and CD8/MHC Class I. CD2 interaction with LFA.-3 is necessary for T lymphocyte activation, binding, and T-helper cell responses. (Fischer et al. 1986; Springer et al. 1987)