Follicular, follicular + diffuse and diffuse categories are distinguished.

Dense infiltrate involving all dermal layers and often extending into the subcutis, diffuse or nodular, “bottom heavy”. No epidermotropism (grenz zone is present), periadnexal and perivascular arrangement.

Follicular and pseudofollicular structures are often present, consisting of centroblasts (at least 20%) and centrocytes (centrocytes dominate). Centroblasts are medium to large cells with a vesicular roudish nucleus containing few distinct nucleoli attached ot the nuclear membrane and basophilic (Giemsa) cytoplasm.

Centrocytes are small or medium sized cells with irregular nuclei showing nuclear clefts.

In addition some immunoblasts and only a few histiocytes are present. About 10 – 40% of the infiltrate are reactive T lymphocytes.

Immunophenotype and genetic features:

• B cell markers: CD20+, CD79a+
• CD5-, CD43-
• expression of surface Ig with monotypic pattern (either lambda or kappa light chains)
• CD21+ dendritic cells within the follicular infiltrates, forming irregular network
• clonal rearrangement of Ig heavy or light chain genes (PCR or Southern blot); negative results does not exclude the diagnosis of the follicular lymphoma.
• bcl2 is usually negative