*6.3.1 Morphology and immunophenotype*

Follicular lymphoma usually shows effacement of nodal architecture by follicular nodules that occupy both cortex and the medulla with two cell types in varying proportions: (1) Centrocytes which are small cells with irregular or cleaved nucleus and scant cytoplasm (2) Centroblasts which are larger cells with open nuclear chromatin, several prominent nucleoli, and modest amounts of cytoplasm. Based on the number of these centroblasts FL is classified into Grade 1, 2, and 3a,b. Grade 1 follicular lymphomas have 0–5 centroblasts per HPF; 6–15 centroblasts per HPF as grade 2, and greater than 15 per HPF as grade 3 follicular lymphomas. Low-grade (grades 1 and 2) follicular lymphomas are composed of a relatively homogeneous population of small cleaved lymphocytes. Grade 3 follicular lymphomas, which by definition have an increased number of large noncleaved cells and have been provisionally subcategorized into 3a and 3b, with the former having a mix of cleaved and large noncleaved cells, and the latter having sheets or large clusters of large noncleaved cells Peripheral blood involvement is seen in about 10% of cases. Bone marrow involvement occurs in 85% of cases and characteristically takes the form of paratrabecular lymphoid aggregates. The neoplastic cells express CD19, CD20, CD10, surface Ig, and BCL6 (B-cell lymphoma 6). BCL2 is expressed in more than 90% of cases. CD5 and cyclin D1 are negative. Ki67 is low [25] (**Figure 3**).
