gained mutations that confer a higher affinity surface antibody towards antigen gains a survival advantage over lower affinity B cell clones and those that have gained deleterious mutations. Cyclic re-entry into the dark zone once again as centroblasts allows a chance for otherwise non-selected B cell mutants to gain more mutations in order to improve affinity towards antigen. Interactions with T cells are also believed to prevent the generation of autoreactive germinal center B cells. At some unclear stage of their centroblast-centrocyte cycling, maturing B cells receive a final differentiation signal to exit the germinal center as an antibody producing plasma cell which are cells that secrete large quantities of antibody or a memory B cell that can be reactivated in subsequent contacts with the same antigen. Selected B cells may also restart the whole cycle of mutative centroblast division and centrocyte selection. In this way the adaptive immune system, in part through these germinal center reactions, can gradually better recognize antigens over time. Morphology at different stages The morphology of GCs is very specific and shows properties which are characteristic for different stages of the reaction. In an early state of the reaction a network of FDCs is fully filled with proliferating B cells. Later at day 4 of the reaction, GCs show a separation of two zones, the dark and the light zone. The former still contains dominantly proliferating and mutating B cells while the latter one is the area of B cell selection. These zones dissolve after 10 days of GC development which ends after about 3 weeks. Medical relevance As germinal centers are important structures of the adaptive immune system, their deregulation is implied in many immune diseases, for example rheumatoid arthritis, immunodeficiency and many lymphomas like DLBCL and Burkitt's lymphoma. See also Lymphatic system Tingible body Mantle zone References External links - "Lymphoid Tissues and Organs: lymph node, germinal centre" Hyperlinked Human Histology Lymphatic system Lymphatic tissue