The lymphatic system is an extensive network of vessels, nodes, and ducts that pass through almost all bodily tissues. It allows the circulation of a fluid called lymph through the body in a similar way to blood.

The lymphatic system is essential for fluid balance, absorption of fatty acids in the stomach, and immune system regulation.

This article details the lymphatic system, its role in the body, and what conditions can impair its function.

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The lymphatic system is a network of vessels, nodes, and ducts that collect and circulate excess fluid in the body.

There are 500–600 lymph nodes throughout the body. These nodes swell in response to infection due to a buildup of lymph fluid, bacteria, or other organisms and immune system cells.

The lymphatic system is part of the immune system. It also maintains fluid balance and plays a role in absorbing fats and fat-soluble nutrients.

The lymph system has three main functions.

Fluid balance

The lymphatic system returns excess fluid and proteins from the tissues that cannot return through the blood vessels. The fluid often collects in the tiny spaces surrounding cells, known as the interstitial spaces. Small lymph capillaries connect these spaces to the lymphatic system.

Around 90% of the plasma that reaches tissues from the arterial blood capillaries returns through the venous capillaries and veins. The remaining 10% travels through the lymphatic system.

A disruption of fluid processing can result in localized swelling, known as lymphedema.

Absorption

The lymphatic system plays a key role in intestinal function. It assists in transporting fat, fighting infections, and removing excess fluid.

Part of the gut membrane in the small intestine contains tiny finger-like protrusions called villi. Each villus contains tiny lymph capillaries, known as lacteals. These absorb fats and fat-soluble vitamins to form a milky white fluid called chyle.

This fluid contains lymph and emulsified fats, or free fatty acids. It delivers nutrients indirectly when it reaches the venous blood circulation. Blood capillaries take up other nutrients directly.

The immune system

The third function of lymph nodes is to defend the body from exposure to potentially hazardous microorganisms, such as infections.

The body’s first line of defense involves:

  • physical barriers, such as the skin
  • toxic barriers, such as the acidic contents of the stomach
  • “friendly” bacteria in the body

However, pathogens often do succeed in entering the body despite these defenses. In this case, the lymphatic system enables the immune system to respond appropriately.

Learn more about how the immune system works here.

How does the lymphatic system fight infection?

The lymphatic system produces white blood cells called lymphocytes. There are two types of lymphocytes: T cells and B cells. They both travel through the lymphatic system.

As they reach the lymph nodes, they come into contact with viruses, bacteria, and foreign particles in the lymph fluid.

Following contact, lymphocytes form antibodies and start to defend the body. They can also produce antibodies from memory if they have already encountered the specific pathogen in the past.

The lymphatic system and the action of lymphocytes form part of the body’s adaptive immune response. These are highly specific and long lasting responses to particular pathogens.

Learn more about antibodies and their role in the body here.

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This shows the lymphatic system. Credit: Yaja’ Mulcare

The lymphatic system consists of lymph vessels, ducts, nodes, and other tissues throughout the body.

Lymphatic vessels collect interstitial fluid and transport it to lymph nodes. These nodes filter out damaged cells, bacteria, and other foreign bodies.

Once this fluid passes out of the lymph nodes, it travels to larger vessels and eventually lymph ducts, which converge in the thoracic duct at the base of the neck.

The thoracic duct returns filtered lymph into the bloodstream.

Other lymphatic tissues

Lymph nodes are not the only lymphatic tissues in the body. The tonsils, spleen, and thymus glands are also lymphatic tissues.

  • Thymus gland: The thymus gland is a lymphatic organ and an endocrine gland behind the sternum. It secretes hormones and is crucial to the production, maturation, and differentiation of immune T cells.
  • Tonsils: The tonsils produce lymphocytes and antibodies. They can help protect against inhaled and swallowed foreign bodies.
  • Spleen: The spleen is not part of the connected lymphatic system, but it is lymphoid tissue. It produces white blood cells and filters the blood to remove microbes as well as old and damaged red blood cells and platelets.
  • Bone marrow: Bone marrow is not lymphatic tissue but is part of the lymphatic system because it is here that the B cell lymphocytes of the immune system mature.

Below is a 3D model of the lymphatic system, which is fully interactive.

Lymph nodes can swell for two common reasons: a reaction to an infection and direct infection of the lymph nodes.

In the former, the lymph nodes react when coming into contact with foreign materials from infected tissue.

Direct infection can cause lymphadenitis. In this, infection causes inflammation in the lymph nodes, and a person will require antibiotic treatment.

Most people who have swollen glands with a cold or flu do not need to contact a doctor.

However, a person should seek medical advice if:

  • lymph nodes stay swollen for longer than 2 weeks
  • a swollen lymph node feels hard or fixed in place
  • swelling accompanies a fever, night sweats, or unexplained weight loss

Swollen lymph nodes can be a symptom of numerous conditions:

  • Glandular fever: Also known as infectious mononucleosis, or mono, this is a viral infection that can cause long lasting swelling, a sore throat, and fatigue.
  • Tonsillitis: This is more common in children than in adults. It occurs when the lymph nodes at the back of the mouth are fighting an infection, usually viral but sometimes bacterial.
  • Pharyngitis: Some people refer to this infection as “strep throat.” It results from a Streptococcus bacterial infection, and it can cause lymph nodes to swell.

Cancer that starts in the lymphatic system is known as lymphoma. It is the most serious lymphatic disease.

Hodgkin lymphoma affects B lymphocytes, a type of white blood cell.

People with Hodgkin lymphoma will have a certain type of lymphocyte present in their blood called Reed-Sternberg cells. Non-Hodgkin lymphoma refers to types that do not involve these cells.

Hodgkin lymphoma can occur across the lymphatic system. However, it most commonly affects lymph nodes in the upper part of the body, such as the neck, chest, and under the arms.

Non-Hodgkin lymphoma can arise from B or T lymphocytes but is most common in B lymphocytes. There are many types of non-Hodgkin lymphomas. These can vary in their location and how aggressive their growth is.

Visit our dedicated cancer hub here.

Cancer cells may spread from their primary site via the lymphatic system. This may cause enlargement of lymph nodes. The most common examples are breast cancer and melanoma.

Learn more about cancer spreading to the lymph nodes here.

If the lymphatic system does not work properly, fluid may not drain effectively. As the fluid builds up, this can lead to swelling in an arm or leg. This is lymphedema.

The skin may feel tight and hard, and skin problems may occur. In some cases, fluid may leak through the skin.

Obstruction can result from:

  • surgery
  • radiation therapy
  • injury
  • a condition known as lymphatic filariasis
  • a congenital disorder

The lymphatic system drains excess fluid that accumulates in bodily tissue, filters out foreign bodies, and transports it back into the bloodstream.

The lymphatic system is a collection of vessels, nodes, and ducts that span most of the body.

Failures of the lymphatic system can cause swelling, venous dysfunction, and life threatening complications.