Lymph nodes are small, bean-shaped glands in the body’s immune system. Hundreds are spread throughout your body, where they help fight illness and infections by filtering harmful substances from lymphatic fluid. You are usually unaware of them unless they become swollen.
The Lymphatic System’s Structure
Lymph nodes are part of the lymphatic system, which also includes lymphatic vessels, lymph, and organs like the spleen and thymus. This system is a drainage network, collecting excess fluid that escapes from blood vessels into tissues and returning it to the bloodstream. This fluid, called lymph, is similar to blood plasma. It contains proteins, minerals, fats, and cells, along with foreign invaders like bacteria.
Lymphatic vessels form a one-way transport system carrying lymph away from tissues. They begin as tiny capillaries that merge into larger vessels. Similar to veins, these larger vessels have valves that prevent the backward flow of fluid. The network directs filtered lymph back into the bloodstream through two large ducts at the base of the neck, maintaining the body’s fluid balance.
This system is an active part of the body’s defense. As lymph flows through the vessels, it passes through strategically placed lymph nodes that act as checkpoints. The nodes filter the fluid before it is returned to circulation. Other lymphatic tissues, like the tonsils and patches in the small intestine, also contribute to this defense process.
Mapping the Major Lymph Node Groups
The body has hundreds of lymph nodes, with clusters found in specific regions. Some nodes are superficial, located just under the skin, while others are deep within the body surrounding organs. The main groups are found in several key areas.
- Head and Neck: Cervical lymph nodes are found along the sides and back of the neck. Submandibular nodes are located under the jaw, and occipital nodes are at the base of the skull. Supraclavicular nodes are situated just above the collarbone.
- Upper Body: Axillary lymph nodes are located deep within the armpits, draining the arm, upper chest wall, and breast area. Inside the chest, mediastinal lymph nodes are positioned between the lungs, filtering lymph from the thoracic organs.
- Abdomen and Pelvis: Mesenteric nodes are in the membranes supporting the intestines, filtering fluid from the digestive tract. Iliac lymph nodes are in the pelvis and drain the pelvic organs. These deep nodes are not palpable and are seen on medical imaging.
- Lower Body: Inguinal lymph nodes are a major cluster in the groin, filtering lymph from the leg, groin, and lower abdominal wall. A smaller group, the popliteal nodes, can be found behind the knee.
Function of Lymph Nodes in Health
Lymph nodes serve as filtration and surveillance centers. Their primary job is to filter lymph fluid, trapping foreign particles like bacteria, viruses, and other pathogens. Macrophages, a type of immune cell in the node, engulf and destroy this debris. This process cleans the fluid before it re-enters the bloodstream, preventing infections from spreading.
Beyond filtration, lymph nodes are hubs for the immune system’s adaptive response. They are packed with specialized white blood cells called lymphocytes, mainly T-cells and B-cells. When pathogens are detected, these cells are activated to mount a targeted defense. Dendritic cells present fragments of the invaders, called antigens, to T-cells to initiate a response.
Once activated, B-cells multiply and produce antibodies, which are proteins that neutralize specific pathogens. T-cells are also activated to either kill infected cells or help coordinate the immune attack. This process of immune cell activation and proliferation causes lymph nodes to swell during an infection.
Clinical Importance of Lymph Node Mapping
The location of swollen lymph nodes, or lymphadenopathy, gives diagnostic clues to healthcare providers. Because nodes filter fluid from nearby tissues, their swelling often points to the site of an infection or inflammation. For instance, an upper respiratory infection frequently causes the cervical lymph nodes in the neck to enlarge. An infection in the leg might lead to swollen inguinal nodes in the groin.
While most cases of lymphadenopathy are due to common infections and resolve on their own, persistent swelling can signal a systemic issue. Autoimmune disorders like lupus or rheumatoid arthritis can cause this. Certain infections such as HIV or mononucleosis can also cause generalized lymphadenopathy affecting multiple node groups.
The lymphatic map is also important in oncology. Cancer cells can break away from a primary tumor and travel through the lymphatic system in a process called metastasis. These cells can become trapped in the nearest lymph nodes. The first node that drains a tumor is the sentinel node, and its condition indicates if the cancer has begun to spread.
Surgeons perform a sentinel lymph node biopsy to check for metastasis. A special dye or a weak radioactive substance is injected near the tumor to identify the sentinel node. The surgeon then removes this node for examination by a pathologist. If the sentinel node is free of cancer, the disease has likely not spread, and more extensive node removal may be avoided. If cancer cells are found, it helps determine the cancer’s stage and plan treatment.