Which immunophenotypic marker is most useful for identifying B-cell acute lymphoblastic leukemia (B-ALL) and distinguishing it from T-cell ALL?

Question

Accepted Answer

D. CD19 and/or CD20 expression. ## Immunophenotypic Classification of Acute Lymphoblastic Leukemia

**Key Point:** CD19 is the most sensitive and specific marker for B-cell lineage in ALL. CD20 is also B-cell associated but appears later in B-cell development. Together, CD19 ± CD20 definitively identifies B-ALL and distinguishes it from T-ALL.

### B-Cell vs. T-Cell ALL Immunophenotype

| Marker | B-ALL | T-ALL | Utility |
| --- | --- | --- | --- |
| **CD19** | **+++ (>95%)** | **−** | **Gold standard for B-cell lineage** |
| **CD20** | ++ (variable) | − | B-cell marker; appears later in development |
| CD7 | − | +++ | T-cell marker; most sensitive |
| CD5 | − (rare) | +++ | T-cell marker; co-expressed with CD7 in T-ALL |
| CD34 | ++ (precursor ALL) | ++ (precursor ALL) | Non-lineage specific; marks immature cells |
| HLA-DR | ++ | − | B-cell and monocytic marker; not T-cell |
| CD10 | ++ (common ALL) | − | B-cell marker; present in ~80% of childhood B-ALL |
| CD13/CD33 | − | − | Myeloid markers; absent in ALL |

**High-Yield:** 
- **B-ALL:** CD19+ (essential), CD20 variable, CD10+ (common ALL subtype), CD34+ (if precursor), HLA-DR+
- **T-ALL:** CD7+ (most sensitive), CD5+, CD34+ (if precursor), HLA-DR−

**Clinical Pearl:** CD19 is present in >95% of B-ALL cases and is retained even in mature B-ALL. It is the most reliable single marker for B-cell lineage assignment. CD20 appears later in B-cell development and may be negative in early precursor B-ALL, making it less sensitive than CD19.

**Mnemonic:** **B-ALL = B19** — CD19 is the marker for B-cell acute lymphoblastic leukemia. Remember: CD19 is present in nearly all B-ALL, while CD20 is variable and appears later.

Answer

A. CD34 expression

Answer

B. HLA-DR expression

Answer

C. CD7 and CD5 co-expression

Which immunophenotypic marker is most useful for identifying B-cell acute lymphoblastic leukemia (B-ALL) and distinguishing it from T-cell ALL?

Explanation

Immunophenotypic Classification of Acute Lymphoblastic Leukemia

B-Cell vs. T-Cell ALL Immunophenotype

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Marker	B-ALL	T-ALL	Utility
CD19	+++ (>95%)	−	Gold standard for B-cell lineage
CD20	++ (variable)	−	B-cell marker; appears later in development
CD7	−	+++	T-cell marker; most sensitive
CD5	− (rare)	+++	T-cell marker; co-expressed with CD7 in T-ALL
CD34	++ (precursor ALL)	++ (precursor ALL)	Non-lineage specific; marks immature cells
HLA-DR	++	−	B-cell and monocytic marker; not T-cell
CD10	++ (common ALL)	−	B-cell marker; present in ~80% of childhood B-ALL
CD13/CD33	−	−	Myeloid markers; absent in ALL