Which immunophenotypic marker pattern best distinguishes B-cell acute lymphoblastic leukemia (B-ALL) from T-cell acute lymphoblastic leukemia (T-ALL)?

Explanation

## Immunophenotypic Distinction: B-ALL vs T-ALL ### Lineage-Specific Markers | Marker | B-ALL | T-ALL | |--------|-------|-------| | **CD19** | **Positive** | Negative | | **CD20** | **Positive** (variable) | Negative | | **CD79a** | **Positive** | Negative | | **CD7** | Negative | **Positive** | | **CD5** | Negative | **Positive** | | **CD2** | Negative | **Positive** | | **CD3** | Negative | **Positive** (cytoplasmic or surface) | | **CD34** | Positive (common ALL) | Positive (T-ALL) | | **TdT** | Positive | Positive | | **HLA-DR** | Positive | Variable | **Key Point:** CD19 and CD20 are B-lineage markers; CD7, CD5, and CD2 are T-lineage markers. The presence of B-lineage markers (CD19, CD20, CD79a) definitively identifies B-ALL, while T-lineage markers (CD7, CD5, CD2, CD3) identify T-ALL. These are the most reliable discriminators. **High-Yield:** In clinical practice, a simple flow cytometry panel checking CD19 (B-cell marker) vs. CD7/CD3 (T-cell markers) rapidly distinguishes B-ALL from T-ALL. Both are TdT-positive and may express CD34, so these markers alone cannot discriminate between the two. **Clinical Pearl:** T-ALL has a worse prognosis than B-ALL in children but similar outcomes in adults with modern chemotherapy protocols. Accurate lineage assignment is critical for risk stratification and treatment planning (e.g., T-ALL may benefit from intensified CNS prophylaxis). **Mnemonic:** **B-ALL = CD19, CD20 (B markers); T-ALL = CD7, CD5, CD2, CD3 (T markers)**