Once released from the bone marrow and thymus lymphocytes begin a life of patrol and respond. This degree of patrol allows rapid response to infectious agents. The lymphatic system is a series of vessels which drain and filter the tissue fluids . Lymphocytes enter the node primarily from the blood via specialised endothelia also within the T areas and leave via the efferent lymphatics. Dendritic cells migrating from the tissues enter the node into the T cell areas.
Germinal centres develop in response to antigen.
Here B cells proliferate in response to antigen on follicular dendritic cells and give rise to progeny with high affinity for antigen through a process called affinity maturation. The spleen is fed by a single artery and does not receive afferent lymphatic drainage. The sheaths are associated with lymphoid follicles of similar organisation to the lymph node. The largest concentration of lymphoid tissue is, however, in the gut and other mucosal areas. Here aggregates of lymphoid tissue, similar to lymph nodes in organisation, constitute Peyer's Patches in the lamina propria of the small intestine, tonsils in the pharynx, and submucosal lymphoid follicles in the appendix and throughout the upper airways.