Lymphatic System

Lymph nodes are pockets of lymph that orchestrate the removal of foreign material (including bacteria, viruses, and cancerous cells) from the lymph. They vary in size from microscopic to about 0.16 in (0.394 cm) in diameter. Some nodes cluster at key sites where the limbs join the torso. Lymph nodes are named after their locations in the body. The nodes at the arm are called axial and brachial, those under the jaw are called subclavian, and those in the groin are called inguinal. Fibrous connective tissue covers the lymphatic tissue inside the lymph node.

Each node, also called a lymph gland, has both arterial blood supply and venous drainage. Lymphocytes drain out of the arteries into the node interior, usually through a high endothelial venule that facilitates their entry. This venule (small vein) derives its name from the higher-than-usual tightly joined endothelial cells that line it.

Before they can enter the lymph node, lymphocytes are carefully selected from other blood cells. They are recognized and distinguished by a lymphocyte-cell surface protein called E-selectin. Receptors on the endothelial cells bind the E-selectin positive lymphocytes and slowly roll them toward a gap between adjacent cells. Then the lymphocyte is fed through this area much the way film is fed into a camera. The lymphocytes emerge on the interior of the node.

The internal lymph node tissue is separated into lobes. The lobe end at the center of the node is called the medulla, whereas the wider lobe end toward the perimeter of the node is called the cortex. The lobe area just next to the cortex is called the paracortex. Surrounding the lobes is an area called the medullary sinus. T cells are concentrated in the paracortex, whereas B cells primarily are concentrated in the cortex in structures called primary follicles. Lymphocytes first travel to the medullary sinus before migrating to the cortical and paracortical regions.

In addition to lymphocytes, several other kinds of antigen-fighting wbcs are contained within the nodes. Macrophages destroy and devour foreign antigens under direction from lymphocytes. Within the cortex, a large wbc called an interdigitating dendritic cell actually gathers the foreign antigen and presents it to the T cells that, in turn, trigger the antigen's destruction. This system is carefully controlled to avoid destroying host cells. Within the paracortex, follicular dendritic cells present antigens to B cells in a region of the follicles called the germinal centers. Within germinal centers, memory B cells are formed that are specifically primed to launch an attack against an antigen if it is encountered again. Like seasoned soldiers who know how to fight a particular enemy, memory B cells are molecularly armed to combat a known antigen.

Foreign antigens are constantly being destroyed; however, when a particularly strong infection occurs, the lymph nodes will sometimes swell with the influx of backup troops (more wbcs) sent in to help fight a particular molecular attacker. Eventually, the lymphocytes leave the node through the efferent lymphatic vessel.