A 6-year-old girl from rural India presents with a 2-week history of fever, bone pain, and easy bruising. Investigations reveal WBC 8,500/µL with 85% blasts, hemoglobin 7.2 g/dL, and platelets 45,000/µL. Flow cytometry shows CD19+, CD10+, TdT+, MPO−. Which single feature most reliably distinguishes common B-cell ALL (cALL) from T-cell ALL (T-ALL) in this age group?

Explanation

## Distinguishing Common B-cell ALL from T-cell ALL ### Immunophenotypic and Clinical Discriminators **Key Point:** CD10 (CALLA, common acute lymphoblastic leukemia antigen) positivity defines common B-cell ALL (cALL), which typically presents with lower WBC counts. T-cell ALL (T-ALL) is CD10− and typically presents with higher WBC counts and mediastinal involvement. ### Immunophenotypic Comparison | Feature | Common B-ALL (cALL) | T-cell ALL (T-ALL) | |---------|---------------------|--------------------| | **CD10 (CALLA)** | Positive (>95%) | Negative | | **CD19** | Positive | Negative | | **CD5** | Negative | Positive | | **CD7** | Negative | Positive | | **TdT** | Positive | Positive | | **MPO** | Negative | Negative | | **Cytoplasm** | Scant | Abundant | ### Clinical Presentation Differences **Common B-cell ALL (cALL):** - Accounts for ~80% of childhood ALL - WBC typically <50,000/µL at diagnosis - Better prognosis (5-year survival >90%) - Rare mediastinal involvement - CD10+ is the defining immunophenotypic marker **T-cell ALL (T-ALL):** - Accounts for ~15–20% of childhood ALL - WBC often >100,000/µL at diagnosis (hyperleukocytosis) - Mediastinal mass in 50–70% of cases (thymic involvement) - Worse prognosis than cALL - CD5+, CD7+ are typical markers **High-Yield:** CD10 positivity is the single most reliable immunophenotypic feature that identifies common B-cell ALL and correlates with better prognosis. Its presence essentially defines cALL. ### Why WBC Count Differs **Clinical Pearl:** T-ALL presents with higher WBC counts due to the thymic origin of T-cell precursors and their tendency toward rapid proliferation. cALL arises from bone marrow precursors and typically has lower WBC counts, making it less likely to cause hyperleukocytosis. **Mnemonic:** **CD10 = Common ALL** — CD10 (CALLA) positivity is the hallmark of common B-cell ALL and the best single discriminator from T-ALL. ### Prognostic Implications - **cALL (CD10+):** Better prognosis; responds well to standard chemotherapy - **T-ALL (CD10−, CD5+, CD7+):** Historically poorer prognosis; requires intensified therapy [cite:Harrison 21e Ch 110; Robbins 10e Ch 24]