A rapidly proliferating B cell (B lymphocyte) found within the germinal centre of a lymph node follicle. Centroblasts are derived from B cells activated by exposure to a specific antigen in the first phase of a humoral immune response; they seed the germinal centres and divide rapidly. As numbers increase their progeny cells divide and undergo subtle genetic changes (see somatic hypermutation) that fine-tune their specificity and enhance affinity for the particular antigen. After a few days, some differentiate into either memory cells or antibody-secreting plasma cells and leave the germinal centre.