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 lymphatic or lymph system involves an extensive network of vessels that passes through almost all our tissues to allow for the movement of a fluid called lymph. Lymph circulates through the body in a similar way to blood.
There are about 600 lymph nodes in the body. These nodes swell in response to infection, due to a build-up of lymph fluid, bacteria, or other organisms and immune system cells.
A person with a throat infection, for example, may feel that their "glands" are swollen. Swollen glands can be felt especially under the jaw, in the armpits, or in the groin area. These are, in fact, not glands but lymph nodes.
They should see a doctor if swelling does not go away, if nodes are hard or rubbery and difficult to move, if there is a fever, unexplained weight-loss, or difficulty breathing or swallowing.
Fast facts about the lymphatic system
- The lymphatic system plays a key role in the immune system, fluid balance, and absorption of fats and fat-soluble nutrients.
- As lymph vessels drain fluid from body tissues, this enables foreign material to be delivered to the lymph nodes for assessment by immune system cells.
- The lymph nodes swell in response to infection, due to a build-up of lymph fluid, bacteria, or other organisms and immune system cells.
- Lymph nodes can also become infected, in a condition known as lymphadenitis.
- If lymph nodes remain swollen, if they are hard and rubbery, and if there are other symptoms, you should see a doctor.
The lymphatic system has three main functions:
- It maintains the balance of fluid between the blood and tissues, known as fluid homeostasis.
- It forms part of the body's immune system and helps defend against bacteria and other intruders.
- It facilitates absorption of fats and fat-soluble nutrients in the digestive system.
The system has special small vessels called lacteals. These enable it to absorb fats and fat-soluble nutrients from the gut.
They work with the blood capillaries in the folded surface membrane of the small intestine. The blood capillaries absorb other nutrients directly into the bloodstream.
The lymphatic system consists of lymph vessels, ducts, nodes, and other tissues.
Around 2 liters of fluid leak from the cardiovascular system into body tissues every day. The lymphatic system is a network of vessels that collect these fluids, or lymph. Lymph is a clear fluid that is derived from blood plasma.
The lymph vessels form a network of branches that reach most of the body's tissues. They work in a similar way to the blood vessels. The lymph vessels work with the veins to return fluid from the tissues.
Unlike blood, the lymphatic fluid is not pumped but squeezed through the vessels when we use our muscles. The properties of the lymph vessel walls and the valves help control the movement of lymph. However, like veins, lymphatic vessels have valves inside them to stop fluid from flowing back in the wrong direction.
Lymph is drained progressively towards larger vessels until it reaches the two main channels, the lymphatic ducts in our trunk. From there, the filtered lymph fluid returns to the blood in the veins.
The vessels branch through junctions called lymph nodes. These are often referred to as glands, but they are not true glands as they do not form part of the endocrine system.
In the lymph nodes, immune cells assess for foreign material, such as bacteria, viruses, or fungus.
Lymph nodes are not the only lymphatic tissues in the body. The tonsils, spleen, and thymus gland are also lymphatic tissues.
What do the tonsils do?
In the back of the mouth, there are tonsils. These produce lymphocytes, a type of white blood cell, and antibodies.
They have a strategic position, hanging down from a ring forming the junction between the mouth and pharynx. This enables them to protect against inhaled and swallowed foreign bodies. The tonsils are the tissues affected by tonsillitis.
What is the spleen?
The spleen is not connected to the lymphatic system in the same way as lymph nodes, but it is lymphoid tissue. This means it plays a role in the production of white blood cells that form part of the immune system.
Its other major role is to filter the blood to remove microbes and old and damaged red blood cells and platelets.
The thymus gland
The thymus gland is a lymphatic organ and an endocrine gland that is found just behind the sternum. It secretes hormones and is crucial in the production, maturation, and differentiation of immune T cells.
It is active in developing the immune system from before birth and through childhood.
The bone marrow
Bone marrow is not lymphatic tissue, but it can be considered part of the lymphatic system because it is here that the B cell lymphocytes of the immune system mature.
Liver of a fetus
During gestation, the liver of a fetus is regarded as part of the lymphatic system as it plays a role in lymphocyte development.
Below is a 3-D model of the lymphatic system, which is fully interactive.
Explore the model using your mouse pad or touchscreen to understand more about the lymphatic system.
The lymph system has three main functions.
The lymphatic system helps maintain fluid balance. It returns excess fluid and proteins from the tissues that cannot be returned through the blood vessels.
The fluid is found in tissue spaces and cavities, in the tiny spaces surrounding cells, known as the interstitial spaces. These are reached by the smallest blood and lymph capillaries.
Around 90 percent of the plasma that reaches tissues from the arterial blood capillaries is returned by the venous capillaries and back along veins. The remaining 10 percent is drained back by the lymphatics.
Each day, around 2-3 liters is returned. This fluid includes proteins that are too large to be transported via the blood vessels.
Loss of the lymphatic system would be fatal within a day. Without the lymphatic system draining excess fluid, our tissues would swell, blood volume would be lost and pressure would increase.
Most of the fats absorbed from the gastrointestinal tract are taken up in a part of the gut membrane in the small intestine that is specially adapted by the lymphatic system.
The lymphatic system has tiny lacteals in this part of the intestine that form part of the villi. These finger-like protruding structures are produced by the tiny folds in the absorptive surface of the gut.
Lacteals 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 is to defend the body against unwanted organisms. Without it, we would die very soon from an infection.
Our bodies are constantly exposed to potentially hazardous micro-organisms, 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 our immune system to respond appropriately.
If the immune system is not able to fight off these micro-organisms, or pathogens, they can be harmful and even fatal.
A number of different immune cells and special molecules work together to fight off the unwanted pathogens.
How does the lymphatic system fight infection?
The lymphatic system produces white blood cells, known as lymphocytes. There are two types of lymphocyte, T cells and B cells. They both travel through the lymphatic system.
As they reach the lymph nodes, they are filtered and become activated by contact with viruses, bacteria, foreign particles, and so on in the lymph fluid. From this stage, the pathogens, or invaders, are known as antigens.
As the lymphocytes become activated, they 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.
Collections of lymph nodes are concentrated in the neck, armpits, and groin. We become aware of these on one or both sides of the neck when we develop so-called "swollen glands" in response to an illness.
It is in the lymph nodes that the lymphocytes first encounter the pathogens, communicate with each other, and set off their defensive response.
Activated lymphocytes then pass further up the lymphatic system so that they can reach the bloodstream. Now, they are equipped to spread the immune response throughout the body, through the blood circulation.
The lymphatic system and the action of lymphocytes, of which the body has trillions, form part of what immunologists call the "adaptive immune response." These are highly specific and long-lasting responses to particular pathogens.
The lymphatic system can stop working properly if nodes, ducts, vessels, or lymph tissues become blocked, infected, inflamed, or cancerous.
Hodgkin lymphoma affects a specific type of white blood cell known as Reed-Sternberg cells. Non-Hodgkin lymphoma refers to types that do not involve these cells.
Cancer that affects the lymphatic system is usually a secondary cancer. This means it has spread from a primary tumor, such as the breast, to nearby or regional lymph nodes.
Localized lymphadenitis affects the nodes near the infection, for example, as a result of tonsilitis.
Generalized lymphadenitis can happen when a disease spreads through the bloodstream and affects the whole body. Causes range from sepsis to an upper respiratory tract infection.
If the lymphatic system does not work properly, for example, if there is an obstruction, fluid may not drain effectively. As the fluid builds up, this can lead to swelling, for example 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, or—rarely—a congenital disorder.
Why do lymph nodes swell?
The "swollen glands," that occur, for example, in the neck during a throat infection, are in fact enlarged lymph nodes.
Lymph nodes can swell for two common reasons:
Reaction to an infection: The lymph nodes react when foreign material is presented to immune cells through the lymph that is drained from infected tissue.
Direct infection of the lymph nodes: The nodes can become infected and inflamed as a result of certain infections that need prompt antibiotic treatment. This is lymphadenitis.
Most people who have swollen glands with a cold or flu do not need to see a doctor.
However, medical advice should be sought if:
- lymph nodes stay swollen for longer than 1 to 2 weeks
- a swollen lymph node feels hard or fixed in place
- swelling is accompanied by fever, night sweats, or unexplained weight loss
Swollen lymph nodes can be symptoms of numerous conditions.
Tonsillitis: This is more common in children than in adults. It occurs when the lymph nodes at the back of the mouth are fighting infection, usually viral, but sometimes bacterial.
Pharyngitis: This bacterial infection is commonly called "strep throat." It is caused by group A streptococcus bacteria, and it can cause lymph nodes to swell.
Children are more prone to swollen lymph nodes because their immune systems are still developing their responses to infectious microbes.
MNT has previously published articles on the following research findings:
In October 2017, researchers found that the brain has lymphatic vessels, allowing it to process "waste" leaked from the blood vessels. This could provide new insight into the relationship between the brain and the immune system.
In June 2015, scientists announced that they had discovered a previously unknown lymphatic system that linked it to the central nervous system (CNS) and the brain.
In May 2015, researchers said that the lymphatic system may play a role in helping the heart to recover after a cardiac arrest.