The CD3 T-cell-co-receptor is a protein complex. In mammals, the complex contains a CD3γ chain, a CD3δ chain, and two CD3ε chains. CD3 is initially expressed in the cytoplasm of pro-thymocytes, the stem cells from which T-cells arise in the thymus. This high specificity, combined with the presence of CD3 at all stages of T-cell development, makes it a useful immunohistochemical marker for T-cells in tissue sections. As like as on T-cells, several surface molecules form the antigen receptor and form a complex on B lymphocytes. The (almost) B cell-specific CD19 phosphoglycoprotein is one of these molecules. CD19 expressed on all stage of B cell, except on the maturation to plasma cells.

By using the specific marker CD3 and CD19, this antibody duo is available to identify the population of T-cell or B-cell.