A 6-year-old boy presents with a 2-week history of fever, petechiae, and hepatosplenomegaly. Peripheral blood smear shows blasts with fine chromatin, prominent nucleoli, and abundant cytoplasm with cytoplasmic vacuoles. Flow cytometry is positive for CD19, CD10, and TdT. Which single feature best distinguishes common ALL (cALL) from T-cell ALL (T-ALL) in this child?

Explanation

## Distinguishing Common ALL (cALL) from T-cell ALL (T-ALL) ### Immunophenotypic Profiles **Key Point:** While morphology and many immunophenotypic markers overlap between cALL and T-ALL, the **T-cell antigen expression pattern** is the most reliable discriminator. | Feature | Common ALL (cALL) | T-cell ALL (T-ALL) | |---------|-------------------|--------------------| | **CD19** | Positive | Negative or weak | | **CD10** | Positive (95%) | Negative or weak | | **TdT** | Positive | Positive | | **CD7** | Negative | Positive (95%) | | **CD5** | Negative | Positive (70%) | | **CD2** | Negative | Positive (80%) | | **CD3** | Negative (cytoplasmic in some) | Positive (surface) | | **Morphology** | Fine chromatin, prominent nucleoli, vacuoles | Similar morphology | | **Cytoplasm** | Abundant, vacuolated | Abundant, vacuolated | ### Why CD7/CD5 Co-Expression is the Best Discriminator **High-Yield:** T-ALL blasts express **T-cell associated antigens** (CD7, CD5, CD2, CD3). The presence of **CD7 and CD5 co-expression on the same blast population** is virtually pathognomonic for T-ALL. In contrast, cALL blasts are **CD7-negative and CD5-negative**, as they are B-cell precursors. **Clinical Pearl:** The absence of CD7 and CD5 co-expression (i.e., their negativity together) in cALL is what distinguishes it from T-ALL. When you see a blast population that is CD7+ and CD5+, you are looking at T-ALL. The converse — CD7-negative, CD5-negative blasts — points to cALL. **Mnemonic:** **T-ALL = T-cell antigens (CD7, CD5, CD2, CD3) PRESENT; cALL = T-cell antigens ABSENT** — the immunophenotype mirrors the cell lineage. ### Why Other Options Are Suboptimal - **Cytoplasmic vacuoles and abundant cytoplasm:** Both cALL and T-ALL can show these morphologic features; not discriminatory. - **CD19, CD10, and TdT positivity:** This describes the B-cell lineage of cALL, but the question already states the child has this profile. The question asks what distinguishes cALL from T-ALL in this context — the absence of T-cell markers is the answer. - **Fine chromatin and prominent nucleoli:** Both cALL and T-ALL share similar morphology; this is not discriminatory. ### Clinical Context The child in the vignette has **CD19+ CD10+ TdT+ blasts**, which is consistent with cALL. The absence of T-cell markers (CD7, CD5, CD2, CD3) confirms cALL rather than T-ALL. If the blasts were CD7+ and CD5+, the diagnosis would shift to T-ALL despite the similar morphology and TdT positivity. [cite:Harrison 21e Ch 110]