A lymph node, or lymph gland, is a small, bean-shaped organ of the immune system and lymphatic system. Lymph nodes are widely distributed throughout the body and are linked by lymphatic vessels, where they function as filters for foreign particles and are key sites for the initiation of adaptive immune responses. They house various immune cells, including lymphocytes such as B cells and T cells, as well as macrophages and dendritic cells.

Structure and Anatomy

Adult humans have approximately 500–600 lymph nodes distributed throughout the body, with clusters found in the neck (cervical), armpits (axillary), chest (mediastinal), abdomen (mesenteric), and groin (inguinal). Anatomy, Lymph Nodes - StatPearls Their size varies from a few millimeters to over a centimeter in diameter.

Each lymph node is enclosed in a fibrous capsule made of dense connective tissue. The substance of the node is divided into an outer cortex and an inner medulla.

Microscopic Anatomy (Histology)

• –Capsule and Trabeculae: The outer capsule extends internally as trabeculae, which provide structural support to the node.
• –Cortex: The outer region just beneath the capsule. It contains lymphoid follicles, which are dense aggregations of B cells. In response to an antigen, these can develop into secondary follicles containing a germinal center, a site of B cell proliferation and differentiation. SEER Training Modules: Lymph Node Structure
• –Paracortex: Located between the cortex and the medulla, this area is rich in T cells and dendritic cells. It is here that dendritic cells present antigens to T cells, initiating a cellular immune response.
• –Medulla: The innermost part of the lymph node, consisting of medullary cords and medullary sinuses. The medullary cords contain antibody-secreting plasma cells, B cells, and macrophages.
• –Sinuses: These are vessel-like spaces that serve as channels for lymph flow. Lymph enters the node via multiple afferent lymphatic vessels into the subcapsular sinus, then flows through the trabecular and medullary sinuses before exiting through a single efferent lymphatic vessel at the hilum.

Function

Lymph nodes perform two primary, interrelated functions: filtration and immune activation.

Filtration

As lymph percolates through the sinuses of the node, macrophages and other phagocytic cells engulf and destroy foreign particles, cellular debris, bacteria, viruses, and cancer cells. This process cleanses the lymph before it is returned to the bloodstream. Lymph node | Definition, Function, & Location

Immune Activation

Lymph nodes are the primary sites for the initiation of an adaptive immune response. Specialized antigen-presenting cells, mainly dendritic cells, migrate from tissues to the lymph nodes carrying antigens from pathogens. In the paracortex, they present these antigens to helper T cells, triggering their activation. Activated helper T cells then help activate B cells in the follicles, leading to their proliferation and differentiation into plasma cells, which produce large quantities of antibodies, and memory B cells, which provide long-term immunity. Anatomy, Lymph Nodes - StatPearls

Clinical Significance

The state of lymph nodes is a crucial indicator of health and disease.

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  Lymphadenopathy: This is the medical term for the swelling or enlargement of lymph nodes. It is a common response to infection (a condition known as reactive lymphadenitis), inflammation, or cancer. Infections like streptococcus throat, mononucleosis, or HIV commonly cause lymphadenopathy.

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  Cancer: Lymph nodes are significant in oncology for two main reasons:

  1. –Lymphoma: Cancers that originate within the cells of the lymphatic system itself, such as Hodgkin's lymphoma and non-Hodgkin's lymphoma.
  2. –Metastasis: Cancers originating in other organs (carcinomas or sarcomas) often spread through the lymphatic system. Cancer cells can become trapped in regional lymph nodes, where they may form secondary tumors. The examination of lymph nodes is a critical part of cancer staging. Understanding the Immune System: How It Works
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  Sentinel Node Biopsy: In the surgical treatment of certain cancers, particularly breast cancer and melanoma, a sentinel node biopsy is performed. The sentinel node is the first lymph node to which cancer cells are most likely to spread from a primary tumor. Its removal and examination help determine if the cancer has metastasized, guiding further treatment decisions.