Chronic lymphocytic leukaemia is characterised by a progressive accumulation of functionally incompetent lymphocytes, usually monoclonal.
Morphology. The bone marrow smear in patients with CLL and its variant B-PLL is hypercellular with monotonous areas of small round cells containing a thin rim of cytoplasm. On higher magnification the lymphocytes can be clearly distinguished from erythroid precursors. The mature lymphocytes of CLL are small round cells with pyknotic nuclei and scant blue cytoplasm. In contrast, the most mature normoblasts have a more clumped nuclear chromatin along with a greater amount of cytoplasm, which is either bluish-red (polychromatophilic normoblast) or reddish (orthochromatic normoblast).
Immunophenotyping will confirm the presence of circulating clonal B lymphocytes, with specific surface markers distinguishing CLL from other B-cell neoplasms such as B-PLL.
In B-PLL, prolymphocytes must exceed 55% of lymphoid cells in the peripheral blood. These cells express pan-B-cell markers and high levels of surface immunoglobulin; they are CD5-negative in two-thirds of cases and almost always negative for CD10. Cases of mantle cell lymphoma masquerading as B-PLL must be excluded, particularly in suspected B-PLL cases that express CD5. The differential diagnosis of B-PLL includes other chronic lymphoid neoplasms with a leukaemic presentation.