Which antigen most likely indicates a mature B-lymphocyte (B cell) phenotype, such as in Chronic Lymphocytic Leukemia (CLL) or lymphoma?

Surface Immunoglobulin is the Most Likely Marker of Mature B-Lymphocyte Phenotype

Surface immunoglobulin (option D) is the most likely marker indicating a mature B-lymphocyte phenotype, such as in CLL or lymphoma.

Understanding B-Cell Markers in Mature B-Cell Neoplasms

When identifying mature B-cell neoplasms like CLL or lymphoma, the expression pattern of surface markers is critical for accurate diagnosis. Let's examine each option:

CD5 (Option A)

• While CD5 is expressed on CLL cells, it is not specific to mature B-lymphocytes
• CD5 is actually a T-cell antigen that is aberrantly expressed on B-cells in CLL and some lymphomas 1
• CD5 alone cannot distinguish mature B-cell neoplasms as it's also found on normal T-cells

CD11c (Option B)

• Not a defining marker for mature B-lymphocyte phenotype
• More commonly associated with hairy cell leukemia and some marginal zone lymphomas
• Not consistently expressed in CLL or most B-cell lymphomas

CD19 (Option C)

• While CD19 is a B-cell marker, it is expressed throughout B-cell development
• Present on both immature and mature B-cells
• Not specific to mature B-lymphocyte phenotype 1

Surface Immunoglobulin (Option D)

• The hallmark of mature B-lymphocytes
• Represents completed B-cell development and is required for antigen recognition
• In CLL, surface immunoglobulin is characteristically present but at low levels 1
• Each clone of leukemia cells is restricted to expression of either kappa or lambda immunoglobulin light chains, demonstrating clonality 1, 2
• Light chain restriction (either kappa or lambda) is a definitive marker of B-cell clonality and maturity 2

Diagnostic Criteria for CLL and Other Mature B-Cell Neoplasms

According to the International Workshop on CLL (IWCLL) and ESMO guidelines, the diagnosis of CLL requires:

• ≥5 × 10^9 B lymphocytes/L in peripheral blood
• Clonality confirmed by flow cytometry showing:
  • Co-expression of CD5 with B-cell antigens (CD19, CD20, CD23)
  • Characteristically low levels of surface immunoglobulin, CD20, and CD79b
  • Light chain restriction (kappa or lambda) 1

Clinical Implications

The presence of surface immunoglobulin with light chain restriction is essential for:

1. Confirming B-cell clonality
2. Distinguishing between different mature B-cell neoplasms
3. Differentiating CLL from other CD5+ lymphoproliferative disorders like mantle cell lymphoma

Common Pitfalls to Avoid

• Don't confuse CD5 expression as specific to B-cells; it's actually a T-cell marker aberrantly expressed on some B-cell malignancies
• Remember that while CD19 identifies B-lineage, it doesn't distinguish mature from immature B-cells
• Surface immunoglobulin levels in CLL are characteristically low compared to normal B-cells, but their presence confirms the mature B-cell phenotype

In summary, while CLL cells express multiple markers including CD5, CD19, CD20, and CD23, the presence of surface immunoglobulin with light chain restriction is the definitive marker of a mature B-lymphocyte phenotype, making option D the correct answer.