Immunophenotypic Markers in Acute Lymphoblastic Leukemia
The correct answer is B. CD19, which is the definitive B-cell lineage marker expressed across all stages of B-cell precursor ALL, representing approximately 75-80% of ALL cases. 1
B-Cell ALL Immunophenotype (75-80% of ALL cases)
CD19 is the hallmark marker for B-cell lineage ALL, expressed consistently across all developmental stages from pro-B-cell through mature B-cell ALL. 1 The NCCN guidelines specifically identify CD19, along with CD22 and CD79a, as the defining B-cell markers used to establish B-lineage ALL diagnosis. 1
Key B-Cell Markers by Maturation Stage:
- Pro-B-cell (Pre-pre-B) ALL: CD19+/CD22+/CD79a+, TdT+, CD10 negative 1
- Common B-cell ALL: CD19+/CD10+ (the "common ALL antigen") 1
- Pre-B-cell ALL: CD19+/CD10+/CD22+/CD79a+ with cytoplasmic immunoglobulins 1
- Mature B-cell ALL: CD19+ with surface immunoglobulins, TdT negative 1
Why the Other Options Are Incorrect
CD7 (Option A) - T-Cell Marker
CD7 is a T-cell lineage marker, not a B-cell marker, and is associated with T-cell ALL which represents only 10-25% of ALL cases. 1 T-cell ALL shows variable expression of CD2/CD5/CD7 along with cytoplasmic or surface CD3. 1
CD13 (Option C) - Myeloid Marker
CD13 is a myeloid-associated antigen, not a lymphoid marker. 2, 3 While CD13 can be aberrantly expressed in 25-45% of B-cell precursor ALL cases (particularly in Philadelphia chromosome-positive ALL at 70.8%), the presence of myeloid markers like CD13 does not exclude the diagnosis of ALL. 1, 4 The NCCN guidelines explicitly state that myeloid-associated markers such as CD13 and CD33 may be expressed in ALL without changing the diagnosis to mixed phenotype acute leukemia. 1
CD45 (Option D) - Pan-Leukocyte Marker
CD45 is a pan-leukocyte marker expressed on most hematopoietic cells and is not specific for ALL diagnosis or lineage determination. 3 While useful in flow cytometry gating strategies, it does not define lymphoid lineage.
Clinical Significance
B-cell lineage determination through CD19 expression is critical because it directs treatment toward ALL-specific regimens requiring 2-3 years of multi-agent chemotherapy with CNS prophylaxis, which differs completely from AML treatment approaches. 2 The distinction cannot be made reliably by morphology alone and requires comprehensive flow cytometric immunophenotyping. 2, 3
Common Diagnostic Pitfall:
The presence of aberrant myeloid antigen expression (CD13, CD33) occurs in 25-45% of precursor B-cell ALL cases and is even higher (70.8%) in Philadelphia chromosome-positive ALL. 1, 4 Do not misdiagnose these cases as mixed phenotype acute leukemia (MPAL) or AML - the diagnosis remains B-cell ALL if CD19 and other B-lineage markers are present. 1