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

Question

Accepted Answer

B. CD19 and CD20 positivity in B-ALL; CD7 and CD5 positivity in T-ALL. ## 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)**

Answer

A. TdT positivity in both; CD34 positivity in both

Answer

C. CD13 and CD33 positivity in B-ALL; CD7 and CD2 positivity in T-ALL

Answer

D. HLA-DR positivity in B-ALL; HLA-DR negativity in T-ALL

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

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

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