The OKT3 antibody is specific for human CD3e, also known as CD3 epsilon, a 20 kDa subunit of the T cell receptor complex along with CD3 gamma and CD3 delta. These integral membrane protein chains assemble with additional chains of the T cell receptor (TCR), as well as CD3 zeta chain, to form the T cell receptor – CD3 complex. Together with co-receptors CD4 or CD8, the complex serves to recognize antigens bound to MHC molecules on antigen-presenting cells. These interactions promote T cell receptor signaling (T cell activation), inducing a number of cellular responses including proliferation, differentiation, production of cytokines or activation-induced cell death. CD3 is differentially expressed during thymocyte-to-T cell development and on all mature T cells.

The OKT3 antibody is a widely used phenotypic marker for human T cells. In addition, as the CD3e subunit of the TCR complex contains intracellular signaling domains, binding of OKT3 antibody to CD3e can induce cell activation. A recent publication of the crystal structure of a CD3e-mitogenic antibody complex provides further insight as to the action of commonly used agonist antibodies, as well as specific epitope-binding data for the widely used human CD3 antibodies OKT3 and UCHT1 (Fernandes, R.A. et al. 2012. J. Biol. Chem. 287: 13324-13335). OKT3 has also been shown to be cross-reactive with Chimpanzee CD3 and has been used for in vitro activation of T cells in this species. Please choose the appropriate format for each application.