CD3 is composed of five different polypeptides ranging from 16‑28 kDa that are associated with the T cell receptor (TCR) complex and serve as its signal transducing element. The CD3/TCR complex is expressed on T cells and thymocytes. Binding of immobilized anti-CD3 can cause T cell activation that leads to any of several consequences, depending on the conditions. Activation by anti-CD3 in the presence of IL 2 has been reported to induce cell death, apparently via apoptosis. ⁴ The antibody can be used to induce cytolytic activity against non-specific targets and also to block TCR-mediated cytolytic killing. ¹ 145-2C11 has been used in a variety of studies concerned with allograft rejection and graft-vs host reaction in mice. ^{6, 7}