TdT (Terminal deoxynucleotidal transferase) (precursor cell marker)

Terminal deoxynucleotidyl transferase (TdT) is a nuclear enzyme used as a marker of lymphoid immaturity. It is characteristically positive in precursor (blast) B- and T-lymphoid cells, helping identify acute lymphoblastic leukemia/lymphoblastic lymphoma and distinguish these from mature lymphoid neoplasms and most myeloid leukemias. TdT is typically absent in mature lymphocytes and plasma cells.

Terminal deoxynucleotidyl transferase (TdT) is a nuclear enzyme expressed predominantly in immature lymphoid cells — lymphoblasts of precursor B and T lineages in bone marrow and thymus. TdT is characteristically positive in acute lymphoblastic leukemia and lymphoblastic lymphoma, and is typically absent in mature B cells, mature T cells, and most myeloid cells.

Terminal deoxynucleotidyl transferase (TdT) is a nuclear enzyme in developing B and T lymphocytes that adds non‑templated nucleotides at V(D)J junctions during antigen‑receptor gene recombination, boosting antibody and T‑cell receptor diversity. Clinically, TdT serves as a diagnostic marker for immature lymphoid cells (e.g., acute lymphoblastic leukemia) and is used in lab assays to label DNA breaks.

Terminal deoxynucleotidyl transferase (TdT) is expressed primarily in immature lymphoid cells—pro‑ and pre‑B and pre‑T cells—during early lymphocyte development and V(D)J recombination in the bone marrow and thymus. It is absent in mature B and T cells. TdT is also commonly detected in lymphoblastic leukemias (ALL), where it serves as a diagnostic marker of lymphoid immaturity.

Leukemia is indicated by abnormal complete blood counts (anemia, thrombocytopenia, leukocytosis or leukopenia), circulating blasts on peripheral smear, and bone marrow with ≥20% blasts (acute leukemia). Other markers include specific cytogenetic/molecular abnormalities (e.g., BCR‑ABL, FLT3, NPM1), elevated LDH and uric acid, abnormal coagulation (in APL), lymphadenopathy, splenomegaly, fever and recurrent infections.

CD69 is widely regarded as the best early activation marker for T cells because it is rapidly upregulated within hours after stimulation. For later or sustained activation, CD25 (IL‑2 receptor α), CD38 and HLA‑DR are informative. Functional assays measuring cytokine production (e.g., IFN‑γ) or proliferation provide complementary confirmation, since no single marker perfectly captures all activation states.