## 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.
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