The OKT3 monoclonal antibody specifically recognizes the CD3 epsilon subunit (CD3e/CD3ε) of the CD3 complex which consists of four transmembrane proteins (γ, δ, ε, ζ) that are associated with the T cell antigen receptor (TCR) to form the CD3/TCR complex. The CD3 complex associates with either TCR αβ or TCR γδ heterodimers that are alternatively expressed by some thymocytes, T cells or NKT cells. The CD3 complex is required for the cell surface expression and signal-transducing functions of the TCR. The CD3 complex is expressed by ~60-85% thymocytes and by all peripheral mature T cells. CD3e is also known as T3E or TCRE. CD3e is a ~20 kDa unglycosylated type I transmembrane protein that is encoded by CD3E which belongs to the immunoglobulin superfamily (IgSF). CD3e has an Ig-like extracellular domain (ECD) and an immunoreceptor tyrosine-based activation motif (ITAM) in its cytoplasmic domain. The OKT3 antibody can reportedly fix complement, stimulate T cell proliferation and cytokine production, and block the binding of other human CD3e-specific antibodies including UCHT1 and SK7.