Cluster of Differentiation 3 （CD3） is a T-cell co-receptor expressed by T-cells in thymus, peripheral lymphoid tissue, blood, and bone marrow, as well as activated natural killer cells. CD3 is specifically expressed by T-cells at all stages of development including T-cell lymphomas and leukemias; therefore, it can be used to classify T-cell neoplasms from B-cell and myeloid neoplasms.
