Figure. 1 Lymphoid follicles: main architectural features and phenotypic properties of the different B cell populations involved.

zone. Finally, at the edge between the GC and the follicular mantle, cells coexpressing slgM and the CD5 antigen (CD5+ B cells) are seen.

FDCs are a unique population of accessory cells found exclusively in follicles where they represent a numerically minor cell population (about 2%) that, nevertheless, plays a crucial role in the development of the GC, as follicles devoid of FDCs are never observed. Also, the nodular dissemination of malignant lymphocytes in GC malignancies is characterized by the clustering of monoclonal B lymphocytes around FDCs. FDCs form a dense meshwork of cytoplasmic processes around lymphoid cells which is most prominent in the light zone but extends its branches also amidst B lymphocytes in the corona. FDCs are unable to phagocytose and use their Fc and C3b receptors to fix and localize Ag-Ab complexes on their surface. These complexes provide a long-standing stimulation of GC B lymphocytes as Ag can be held on the FDC surface for more than a year.

Tingible body macrophages are large macrophages that owe their name to their ability to actively phagocytose apoptotic lymphoid cells (the 'tingible bodies' observed in their cytoplasm are apoptotic bodies) and, scattered among proliferating centroblasts, give the typical starry sky appearance to developing GCs. T cells represent about 5% of the GC cellular composition. Although infrequent, they appear to play a crucial role in the generation of the GC, as GCs are virtually absent in congenitally athymic mice; this deficiency can be reversed by the transfer of T cells. T cells are very rare in the dark zone; they are more abundant in the light zone and are Ag specific. Nearly all GC T cells have the CD4' helper phenotype, but exhibit some unusual features, such as the expression of CD57 and the inability to produce interleukin 2 (IL-2).

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