*6.2.1 Morphology and immunophenotype*

Peripheral blood contains large numbers of small round lymphocytes with scant cytoplasm. Some of these cells are usually disrupted in the process of making smears, producing smudge cells. Prolymphocytes can also be circulating based on which it is classified as CLL or CLL/PLL or PLL (Prolymphocytic leukemia). The bone marrow is almost always involved by interstitial infiltrates or aggregates of tumor cells. Infiltrates are also virtually always seen in the splenic white and red pulp and the hepatic portal tracts. Lymph nodes are diffusely effaced by predominantly small lymphocytes with mild irregular nucleus, condensed chromatin, and scant cytoplasm along with proliferation centers, which contain aggregates of mitotically active larger activated lymphocytes. (**Figure 2**).CLL/SLL has a distinctive immunophenotype. The tumor cells express the pan B-cell markers CD19 and CD20, CD23, CD5, LEF1 (Lymphoid Enhancer Binding Factor 1), CD200. Negative for SOX11 (SRY (sex determining region) - Box 11), CD10.
