Tlymphoblasts

T-lymphoblasts are immature T lymphocytes derived from thymic precursors that proliferate in the normal process of T-cell development. They are the malignant cell type in T-cell acute lymphoblastic leukemia (T-ALL) and, less commonly, in thymic lymphoblastic lymphoma. In disease, T-lymphoblasts can be found in bone marrow, peripheral blood, and mediastinal masses, reflecting widespread involvement.

Morphologically, T-lymphoblasts are typically medium to large with a high nucleus-to-cytoplasm ratio, finely dispersed or coarse chromatin, and one or more prominent nucleoli. The cytoplasm is generally scant and basophilic. The cells often appear as a uniform population on routine stains, though some degrees of size variation may be present.

Immunophenotypically, T-lymphoblasts express markers of the T-cell lineage. A hallmark is TdT (terminal deoxynucleotidyl transferase) positivity, reflecting their lymphoid origin. They commonly express cytoplasmic CD3 and other T-lineage markers such as CD7 and CD5. CD1a, CD4, and CD8 expression can be variable and depend on maturation stage; some blasts may also express CD2 or CD34, particularly in early precursor forms. The immunophenotype is essential for distinguishing T-ALL from B-ALL or myeloid leukemias.

Clinical significance and diagnosis hinge on the detection of TdT-positive, T-lineage blasts by flow cytometry or immunohistochemistry in combination with morphology. Cytogenetic and molecular studies (for example, NOTCH1 mutations and TCR gene rearrangements) provide prognostic information and help guide therapy in T-ALL.