A lymph node is an immune system organ that is widely distributed throughout various places in the body. There are about 500-600 nodes in an individual adult, with clusters of lymph nodes found in the underarms, groin, neck, chest, and abdomen. The lymphatic system as a whole is responsible for acting as the body’s primary mechanism of defense. Each node is oval-shaped, and measures between a few millimeters and a few centimeters long. They are linked to one another by lymphatic vessels and generally packed tightly with lymphocytes and macrophages within them, as their primary function is to filter out foreign particles. Lymph nodes also have a significant clinical role in that they become inflamed when an infection is present. The increased numbers of immune system cells fighting the infection will make the node expand and become “swollen.”
Each lymph node is filled with a type of fluid called lymph, which houses lymphocytes that are continuously recirculated throughout the bloodstream. Antigens and other molecules found on bacterial cell walls can be taken up into the lymph system and further into nodes. As a response, the lymphocytes will create an antibody to travel into the bloodstream and target the antigen. Then other cells are created to fight the antigen and will in turn travel to the lymph nodes.
Each node is surrounded by a fibrous capsule on the outer surface that extends into the node itself to form trabeculae. The substance within the lymph node is divided into two parts: the outer cortex and the inner medulla. The cortex is mainly made up of B cells that are arranged as follicles. When lymph flows through the subcapsular sinus, it drains into trabecular sinuses and then flows into the medullary sinuses. The medullary layer of a lymph node is made up of cords and sinuses. Lymphatic tissue, plasma cells, macrophages, and B cells make up the strands of the medullary cords. The sinuses are vessel-like spaces that contain immobile macrophages and reticular cells, as well as separate the cords from one another.
Also within each lymph node are bunches of thin fibers that make up the reticular network. The reticular network stores white blood cells and provides structural support for the lymph node. It also gives a surface for various dendritic cells and macrophages to attach to.
The spleen and tonsils are large lymphoid organs that serve similar functions to lymph nodes, though the spleen filters blood cells rather than lymph.