Lymph nodes are small, bean-shaped organs that filter lymph fluid, a clear or yellowish fluid circulating throughout the lymphatic system. They house immune cells that identify and remove foreign substances, damaged cells, and cancer cells from the lymph fluid before it returns to the bloodstream.
The Architecture of Lymph Nodes
Each lymph node is enclosed by an outer shell called a capsule, which extends inward to divide the node into compartments. Just beneath this capsule are subcapsular sinuses, where lymph fluid enters the node. The lymph node is broadly divided into an outer cortex, a middle paracortex, and an inner medulla.
The outer cortex contains lymphoid follicles, which are dense collections of primarily B lymphocytes, also known as B cells. These follicles can be either primary, meaning they are dormant, or secondary, which are active and contain a pale center called a germinal center. The paracortex, located deeper than the cortex, is largely populated by T lymphocytes, or T cells, and dendritic cells.
High endothelial venules, blood vessels, are present in the paracortex, allowing B and T cells to enter the lymph node from the bloodstream. The innermost region, the medulla, consists of medullary cords and medullary sinuses. Medullary cords are packed with lymphocytes and plasma cells, while medullary sinuses are vessel-like spaces containing reticular cells and macrophages.
Immune Activity Within Zones
Immune responses are initiated within the cortex of the lymph node. Here, B cells within the lymphoid follicles can be activated by antigens, which are often presented by follicular dendritic cells. Once activated, these B cells proliferate and undergo changes within the germinal centers of secondary follicles to produce antibodies with improved antigen-binding capabilities. These germinal centers are also where B cells differentiate into antibody-secreting plasma cells or memory B cells, which are important for long-term immunity.
In the paracortex, T cells encounter antigens presented by dendritic cells, leading to their activation and proliferation. This interaction is a step in initiating adaptive immune responses. Activated T cells can then differentiate into various types, such as helper T cells or cytotoxic T lymphocytes. Both activated B and T lymphocytes eventually migrate to the medulla. The medullary cords serve as a location for activated antibody-secreting plasma cells and macrophages, while the medullary sinuses facilitate the flow of filtered lymph and the exit of activated immune cells from the node.
Lymph Nodes and Your Health
The proper functioning of lymph node zones is important to maintaining overall health and immune responses. Lymph nodes filter lymph and house white blood cells that fight infections. When the body encounters a pathogen, the immune cells within the lymph nodes multiply to fight it, often leading to swelling. This swelling, known as lymphadenopathy, is a common sign that the immune system is clearing an infection.
Lymph nodes also aid in developing immunity after vaccination. Vaccines often introduce antigens, transported to the lymph nodes. Within these nodes, immune cells are activated and trained to recognize the specific antigen, leading to the production of antibodies and memory cells that provide long-term protection against future infections. Lymph nodes also perform immune surveillance against abnormal cells, including cancer cells. Cancer cells can travel through the lymphatic system and become trapped in lymph nodes, which can indicate the spread of cancer and influence treatment strategies.