CD3 is composed of five invariant polypeptide chains that associate to form three dimers. The five invariant chains of CD3 are labeled gamma, delta, epsilon, zeta, and eta. Each of these polypeptides contains at least one immunoreceptor tyrosine-based activation motif (ITAM) These chains associate with the T cell receptor (TCR) and the ζ-chain to generate an activation signal in T lymphocytes.