[Follicular and follicle center-cell lymphomas: A family between kinship and diversity]

Follicular lymphoma (FL) is the second most common B-cell lymphoma in adults. It is classically characterized by a B-cell proliferation of follicular architecture consisting of a mixture of centrocytes and centroblasts. FL cells have a germinal center phenotype and carry the t(14;18)/IGH::BCL2 translocation, the hallmark of FL in 90% of cases. Nevertheless, FL may display particular morphological and molecular characteristics recognized as variants or distinct entities in both the WHO-HAEM5 and International Consensus Classification (ICC). One of the major changes in the WHO-HAEM5 is that the grading of low-grade FL is no more mandatory due to the merging of grades 1, 2 and 3A into a "classic FL" (cFL) subtype, whereas FL with unusual cytological features (uFL) should be identified as a specific variant. uFL is characterised by a proliferation of large centrocytes or small centroblasts/blastoid cells, which do not fit the cytological criteria for grade 3A or 3B. FL grade 3B (which remains unchanged in the ICC) is renamed "follicular large B-cell lymphoma" in the WHO-HAEM5. Duodenal-type FL still remains as a specific subtype characterized by an indolent disease localised to the digestive tract, and harboring a BCL2 rearrangement (BCL2-R). Both current classifications recognize some categories of BCL2-R negative FL including the "predominantly diffuse FL" (WHO-HAEM5) (overlapping with the "BCL2-R negative/CD23 positive follicle center lymphoma" according to the ICC) often presenting in the inguinal region with an indolent course; the paediatric-type FL and the testicular FL affecting children or young adults. The recently described "follicle center lymphoma of the lower female genital tract" shows some similarities with primary cutaneous follicle center lymphoma.