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These undergo cell cycle proliferation into lymphocytes that are able to specifically detect and eliminate pathogens associated with that antigen, through various methods such as cytotoxic action (T cells) and antibody production (B cells). There, antigen-presenting cells called dendritic cells present the antigen molecule to naive B and T lymphocytes. These antigens may be taken by cells into the lymph nodes. Molecules called antigens are found on bacteria cell walls, the cell walls of virus-infected cells, or even chemical substances and toxins secreted from bacteria. These lymphocytes are continuously recirculated through the lymph nodes and the bloodstream. Lymph nodes are the primary site for antigen presentation and activation in adaptive immune response in B and T lymphocytes. The hilum also contains the blood supply of the lymph node. Each lymph node has an afferent lymph vessel that directs lymph into the node, and an efferent lymph vessel called the hilum that directs lymph out of the node at the concave side of the node. Lymph fluid flows into and out of the lymph nodes via the lymphatic vessels, a network of valved vessels that are similar in structure to cardiovascular veins. The lymphatic system: This diagram shows the network of lymph nodes and connecting lymphatic vessels in the human body. The sinuses are partially divided by capsule tissue, which causes lymph fluid to flow around the nodules in each sinus cavity on their way through the node. Inside each sinus cavity is a nodule, a smaller, denser bundle of lymphoid tissue that usually contains a germinal center, the site of B cell proliferation during antigen presentation. The medulla contains plasma cells, macrophages, and B cells as well as sinuses, which are vessel-like spaces that the lymph flows into. The cortex is mainly composed of clusters of B cells in the outer layers and T cells in the inner layers, and may also contain antigen-presenting dendritic cells. Each lymph node is surrounded by a fibrous capsule that encircles the internal cortex and medulla. Humans have approximately 500–600 lymph nodes, with clusters found in the underarms, groin, neck, chest, and abdomen. Lymph nodes are found throughout the body, and are typically 1 to 2 centimeters long.
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Lymph node structure: This diagram of a lymph node shows the outer capsule, cortex, medulla, hilum, sinus, valve to prevent backflow, nodule, and afferent and efferent vessels. Lymph nodes are small oval-shaped balls of lymphatic tissue distributed widely throughout the body and linked by lymphatic vessels.