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A centroblast generally refers to an activated B cell that is enlarged (12-18 micrometer) and proliferating in the germinal center of a secondary lymphoid follicle. Centroblasts form following exposure to follicular dendritic cell cytokines, such as IL-6, IL-15, 8D6, and BAFF and stimulation from helper T cell interactions between their CD40 ligand and the B cell CD40 induces centroblasts to express activation-induced cytidine deaminase, leading to somatic hypermutation.
Centroblasts do not express immunoglobulins and are unable to respond to the follicular dendritic cell antigens present in the secondary lymphoid follicles.