Lymph nodes are bean-shaped structures that cluster along the lymphatic vessels in the inguinal, axillary, and cervical regions. Each node is divided into compartments by a capsule that extends trabeculae inward.
From a histological perspective, lymph nodes can be split into two main areas: the superficial cortex and the deep medulla. The outer cortex is populated by dendritic cells, macrophages, and B lymphocytes, which are densely packed into follicles. When these B-lymphocytes are presented antigens by dendritic cells, they mature into plasma cells that produce antibodies and memory B cells. The inner cortex mainly consists of T-cells and dendritic cells. The dendritic cells activate the T cells. These activated B and T lymphocytes then enter the medulla, from where they are distributed throughout the body.
Lymph flows in one direction through a lymph node. It enters via several afferent lymphatic vessels, which penetrate the node's convex surface at various points. These vessels have valves that direct the lymph inward toward the center of the node. Inside the node, the lymph enters sinuses – irregular channels filled with branching reticular fibers, lymphocytes, and macrophages.
Lymph travels from the afferent vessels into the subcapsular sinus, beneath the node's capsule, then through trabecular sinuses parallel to the trabeculae, and into medullary sinuses. The lymph finally drains into one or two efferent lymphatic vessels, which are broader and less numerous than the afferent vessels. These vessels contain valves that open away from the node's center, carrying lymph having B lymphocytes, antibodies secreted by plasma cells, and activated T cells out of the node.
These efferent vessels emerge from one side of the node at a hilum, a slight depression where blood vessels also enter and leave the node. Lymph nodes function as filters, trapping foreign substances within the sinuses' reticular fibers. Macrophages and lymphocytes destroy these substances.
The lymph, as a result, gets filtered within the lymph nodes. Given the many afferent vessels bringing lymph into a node and only one or two efferent vessels taking lymph out, the flow within the nodes is slow, allowing more time for filtration. In addition, all lymph passes through multiple nodes on its journey through the lymph vessels, ensuring multiple filtering events before it returns to the blood.
