Morphology of Lymphocytes in CLL Peripheral Blood
The peripheral blood in CLL shows a predominance of small, morphologically mature lymphocytes, though it is common to find admixtures of larger or atypical cells including prolymphocytes. 1, 2, 3
Core Morphologic Features
The characteristic morphologic appearance includes:
- Small, mature-appearing lymphocytes that predominate in the blood smear 1, 2, 4
- Admixtures of larger or atypical cells are commonly present, including cells consistent with prolymphocytes 3, 4
- Up to 55% prolymphocytes is still consistent with CLL diagnosis (above this threshold suggests prolymphocytic leukemia) 4
- Occasional lymphoplasmacytic and cleaved cells may be seen, particularly in "atypical CLL" variants 5
Morphologic Variants and Clinical Significance
Approximately 15% of patients show atypical morphology either at diagnosis or during disease progression: 5
- CLL/PL variant: >10% circulating prolymphocytes 5
- "Atypical" CLL: >15% lymphoplasmacytic and cleaved cells 5
- These atypical morphologies correlate with disease progression, advanced stage, trisomy 12, p53 abnormalities, and unmutated immunoglobulin genes 5
Essential Diagnostic Context
While morphology is characteristic, it must be confirmed with immunophenotyping: 1, 2, 3
- The composite immunophenotype (CD5+, CD19+, CD20+ low, CD23+, surface Ig low, CD79b low, FMC7-) distinguishes CLL from other CD5+ B-cell lymphomas 1, 2
- Critical pitfall: Mantle cell lymphoma must be excluded, as it can appear morphologically similar but requires different management—absence of cyclin D1 expression or t(11;14) translocation is essential to confirm CLL 2, 3
Diagnostic Threshold
The diagnosis requires ≥5,000 mature-appearing lymphocytes/μL (5 × 10⁹/L) sustained in peripheral blood, combined with the characteristic morphology and immunophenotype. 1, 2, 3, 6