TdT, which stands for terminal deoxynucleotidyl transferase, is a type of DNA polymerase that does not need a template and adds nucleotides in a random manner to single-stranded DNA. This randomness contributes to acquired immunity by enhancing the diversity of antigen receptors and supporting the production of immunoglobulins and T-cell antigen receptors. TdT is typically expressed in lymphoid progenitor cells found in the thymus and tonsils. The presence of anti-TdT serves as a helpful marker for lymphoblastic lymphomas and has been seen in certain instances of acute myeloid leukemia.