The 145-2C11 monoclonal antibody reacts with mouse CD3ε, a 20 kDa transmembrane cell-surface protein that belongs to the immunoglobulin superfamily. CD3ε is one of five polypeptide chains that combine to form the TCR complex. CD3ε is expressed on T lymphocytes, NK-T cells, and to varying degrees on developing thymocytes. CD3 plays roles in TCR signaling, T lymphocyte activation, and antigen recognition. The 145-2C11 antibody has been shown to induce T lymphocyte activation, proliferation, and apoptosis in vitro via binding and stimulating the TCR. When used in vivo the antibody is reported to produce T cell activation, anergy, or death. Activation of resting T cells in vivo results in cytokine release and subsequent toxicity caused by Ab-mediated cross-linking of T cells and Fcγ receptor (FcR)-bearing cells. For this reason, the non-FcR-binding 145-2C11 f(ab')2 fragments are more commonly used for in vivo applications.  The 145-2C11 antibody has been reported to block the binding of the 17A2 antibody to CD3ε⁺ T lymphocytes.