CD3 is initially expressed in the cytoplasm of pro-thymocytes, the
stem cells from which T-cells arise in the thymus. The pro-thymocytes
differentiate into common thymocytes, and then into medullary
thymocytes, and it is at this latter stage that CD3 antigen begins to
migrate to the cell membrane. The antigen is found bound to the
membranes of all mature T cells, and in virtually no other cell type,
although it does appear to be present in small amounts in Purkinje
cells. 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. 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.
Source: http://en.wikipedia.org/wiki/CD3_(immunology)
