Organization and cellular composition

GCs are placed eccentrically within secondary follicles and have a zonal structure (Figure 1). The upper pole of a GC possesses a prominent crescentlike cap of B mantle lymphocytes: this area is defined as the light zone because of the weak basophilia of the cells. At the opposite (lower or basal) pole, the B mantle may be reduced to a tiny rim of a few cells encompassing an area of highly basophilic cells: the dark zone. The cellular organization of a GC is very composite: B, T and accessory cells have a specific distribution in the different zones of the GC.

Different populations of B lymphocytes form the major cellular component of a GC. According to the level of activation and differentiation, GC B cells have specific morphological, phenotypical and molecular features and occupy different areas, but, in spite of these differences, have two common features. First, they are unable to bind to postcapillary high endothelial venules (HEVs) and lack the normal migratory mechanism that most small lymphocytes possess. Second, they strongly express the adhesion molecules responsible for homotypic adhesion of lymphoid cells, CDlla/CD18 and their natural ligand ICAM 1 (CD54). These properties suggest that GC B cells behave as sessile elements and that Ag-recruited B lymphocytes become transiently segregated into lymphoid follicles in order to properly respond to the stimuli.

The dark zone is made up by centroblasts, a highly proliferating pool of large to medium-sized immature-looking cells with nucleoli and a sparse, intensely basophilic cytoplasm. Centroblasts are surface (s) immunoglobulin (Ig)D~, slgM weakly positive and express major histocompatibility complex (MHC) class II molecules, CD19 and low amounts of CD10 and CD38. The light zone is chiefly made up by centrocytes, B lymphocytes of a more mature appearance with indented (cleaved) nuclci and a pale cytoplasm. Centrocytes are frequently IgG or IgA , CD10+, CD38 f. Further, a few immunoblasts and rare plasma cells are also observed within the light

Ccnlfoblasts

Ccnlfoblasts

Centfoeyi«

CDS' Bcens

Centfoeyi«

CDS' Bcens

Centroblasls

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