The germinal centers, or lymphoblastoid centers, usually appear after the body has been stimulated by antigens such as viruses and bacteria. The germinal center is often a special structure within a lymphoid follicle (a lymphocyte that is closely related to immune function) and is a sign that the lymphoid follicle has been stimulated by an antigen. When lymphocytes are produced in the central region of stimulated lymphoid follicles and lymphoid nodules, lymphoblasts gradually divide in the central part of the lymphoid nodules, and the resulting lymphocytes can be pushed to the periphery of the lymphoid nodules, while lymphoblasts gradually occupy the central part, which is generally referred to as the germinal center. When the body encounters a pathogen, B cells enter the germinal center and evolve within it to produce structures with a high affinity for antibodies against the pathogen.