The immunohistochemical detection of CD3 localizes the cytoplasmic component of the CD3 protein. Anti-CD3 is considered a pan-T cell marker and reacts with an antigen located on the surface and in the cytoplasm of T lymphocytes. Anti-CD3 is often used to identify immature and mature T-cell malignancies. With high specificity and low sensitivity, it is combined with CD45RO to improve diagnostic accuracy.