The lymphatic system forms the immunological fundament for the body's defence system. A circulating pool of lymphocytes ensures a continuous immunologic surveillance of the entire body in order to protect against harmful invasion. These cells circulate between various lymphoid tissues via lymph and bloodstream. When certain lymphoid cells encounter their determined target antigens, these cells return to relevant lymphatic organ or tissue to differentiate (specialise) and proliferate (multiply).
Lymph nodes and lymphatic vessels are present at various locations throughout the body. Lymphatic vessels convey fluid from extracellular spaces to the circulatory system (blood vessels and lymphatic vessels). The walls of lymph capillaries are permeable and allow for the passage into the lymphatic flow for relatively large substances, such as cells and bacterial antigens. As the lymph passes through lymph nodes, specific monitoring cells present there can detect passing antigens and initiate a suitable immune response.
Lymph nodes are aggregates of lymphocytes organized in a reticular network consisting of cells and fibers enclosed by a dense connective tissue capsule. From the capsule, connective tissue called trabeculae penetrate into the organ and provide support for blood vessels entering and leaving the nodes. The lymph nodes lymphatic fluid enters the node through afferent lymphatic vessels and eventually exit through an efferent lymphatic vessel. While travelling from the afferent to the efferent vessel, the lymphatic fluid passes through so called sinuses. These sinuses are located within the lymph node and hamper the flow through the node to enhance filtration of lymphatic fluid and potential antigen detection.
Underneath the capsule two regions can be identified based upon colour, one is the darkly stained region termed the cortex, and the other, which stains lighter, is termed the medulla. The cortex is found just underneath the capsule and within this area multiple spherical aggregations of lymphocytes (lymphoid follicles) are present. Lymphoid follicles consisting of only small, dark lymphocytes (follicle cells) are termed primary follicles. If the central region of a lymphoid follicle contains larger, paler cells, this region is known as a germinal center (GC). Germinal centers are surrounded by a darker outer ring called mantle zone and together they are termed secondary follicles. Secondary follicles are formed when a lymphocyte that has recognized an antigen returns to a primary follicle and starts multiplying. Lymph follicles are absent in the region where the cortex layer meets the medulla since this area is composed mainly of T-cells.