CD3 (Cluster of Differentiation 3) is a complex of proteins that associates directly with the T-cell antigen receptor (TCR). 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. CD3 is involved in T-cell development and survival. It is expressed on T cells in thymus, peripheral lymphoid tissue, blood and bone marrow. CD3 is a commonly used marker for identification of T-cell and T-cell derived malignancies.

CD3 is first detectable in early thymocytes and its appearance probably represents one of the earliest signs of commitment to T cell lineage. In cortical thymocytes, the antigen is predominantly present as an intracytoplasmic constituent.