Pathobiology and diagnosis of follicular lymphoma.

Follicular lymphoma is one of the most frequent lymphomas in the western hemisphere. The diagnosis of this lymphoma is based primarily on morphology and immunohistochemistry. Although the basic diagnostic principles have been unchanged for decades, the concept of lymphoma pathogenesis and subtyping is still being developed. Most importantly, the grading of follicular lymphoma and the definition of transformation continue to be controversial. Early lesions of follicular lymphoma can be identified using immunohistochemistry, which presents a challenge to diagnostic histopathologists, but can by contrast help us to understand the pathogenesis of the disease. Histopathologists should be aware of variants of this lymphoma that present with specific histologic and clinical features, such as pediatric follicular lymphoma.