Test Details: CD3 is a protein complex and T cell co-receptor that is involved in activating both the cytotoxic T cell and T helper cells. It is first expressed in the cytoplasm of developing T cells, then as they mature it moves to the membrane. The antigen is found bound to the membranes of all mature T-cells, this presence of CD3 at all stages of T-cell development, makes it a useful immunohistochemical marker for T-cells in tissue sections. The antigen remains present in almost all T-cell lymphomas and leukaemias, and can therefore be used to distinguish them from superficially similar B-cell and myeloid neoplasms.