Most people know that their lymph nodes can become swollen when they have a throat infection, but why does this happen and what else is there to know about the body's lymphatic system?
The lymphatic system, or lymph system, is similar in many ways to the blood circulatory system, in that it involves an extensive network of vessels that traverse almost all our tissues to allow for the movement of a fluid called lymph. This fluid drains through these lymphatic vessels in a way that is very similar to the return of blood along the veins back to the heart.
Use this page to find out more about this essential part of our immune system and the other roles of the lymphatic system.
At the end of some sections there are links to recent developments concerning the lymphatic system and its diseases, covered by MNT's news stories. Also look out for links to more detailed information on conditions associated with the lymphatic system.
Here are some key points about the lymphatic system. More detail and supporting information is in the body of this article.
- The lymphatic system has three main roles: it is part of our immune system, maintains fluid balance and is essential for the absorption of fats and fat-soluble nutrients.
- Lymph vessels drain fluid from virtually all our tissues to control fluid balance and to deliver foreign material to the lymph nodes for assessment by immune system cells.
- The lymph nodes swell in response to infection - so-called swollen glands - due to a build-up of lymph fluid, bacteria or other organisms and immune system cells.
- Lymph nodes may also become swollen due to direct infection and, rarely, cancer or other diseases or conditions.
- Lymph nodes are responsible for filtering lymph and providing part of the adaptive immune response to new pathogens - the part of our immunity that has a long "memory."
- Disorders of the lymphatics include lymphedema, a form of swelling occurring when lymph has failed to drain through the lymph vessels.
- Swollen lymph nodes can indicate a response to foreign material such as from a nearby infection - this process is known as reactive lymphadenopathy.
- Lymph nodes can also become infected themselves, a condition known as lymphadenitis.
- If swollen lymph nodes do not return to their normal size, are hard or rubbery and difficult to move, are accompanied by fever, unexplained weight-loss, or difficulty breathing or swallowing, a check-up from a doctor is needed.
What is the lymphatic system?
The lymphatic system consists of organs, a fluid called lymph and transportation vessels.
The lymphatic system has three main functions:1,2
- Maintaining the balance of fluid in the blood versus the tissues (fluid homeostasis)
- Forming part of the body's immune system and helping defend against foreign bodies such as bacteria
- Facilitating absorption of fats and fat-soluble nutrients in the digestive system.
With regard to absorption from the gut, the lymphatic system has special small vessels called lacteals that are responsible for taking up fats and fat-soluble nutrients.
These work alongside blood capillaries in the folded surface membrane of the small intestine (the blood capillaries take up the other nutrients directly into the bloodstream).
On the next page, this absorptive role of the lymphatic system is explored briefly while the system's roles in fluid balance and immunity are explained in easy-to-follow detail. Or continue below for the anatomy and workings of the lymphatic system.
Scientists announced in June 2015 that they had unearthed a previously unknown lymphatic system for the central nervous system, "overturning the textbooks."
Researchers publishing in May 2015 said they had improved the scientific understanding of the way lymphatic vessels develop.
Researchers behind a December 2014 study found insights into the drivers of gut lymphatic vessel development.
Lymphatic system organs and anatomy
The lymphatic system comprises lymph vessels, ducts and nodes, as well as other tissues as described below. The lymphatic system works in a similar way to the blood circulatory system, and in close parallel as blood returns to the heart and lymph drains towards the heart.
The lymphatic vessels form a network of branches that infiltrate almost all the body's tissues. These vessels are an accessory to the veins for returning fluid from the tissues.2-6
Its resemblance to the blood circulation system specifically relates to the venous return of blood to the heart. There is no active "pumping" of lymph - a clear fluid derived from blood plasma - but the lymph is pushed back from the peripheries to the center in a way similar to how blood is returned to the heart.
Lymphatic fluid moves through the vessels by being squeezed when we consciously use our skeletal muscles, and by the movement of the smooth muscles when we breathe or perform other involuntary actions. The properties of the lymph vessel walls and the valves help control the movement of lymph.
Like veins, lymphatic vessels have regular valves inside them to stop the backflow of fluid. Lymph is drained progressively towards the larger and larger vessels until it reaches the two main channels, the lymphatic ducts in our trunk, where filtered lymph fluids can be returned to the venous blood.
The lymphatic system's vessels branch through junctions called lymph nodes. These nodes are often referred to as glands, but they are not true glands as they do not form part of the endocrine system.
Lymph nodes are not the only lymphatic tissues in the body - the tonsils, spleen and thymus gland are also lymphatic tissues.
Tonsils are located at the back of the mouth and are responsible for producing lymphocytes (a type of white blood cell) and antibodies. Their position - hanging down from a ring forming the junction between the mouth and pharynx - is strategic for protection against inhaled and swallowed foreign bodies. The tonsils are the tissues affected by tonsillitis, an inflammatory condition common in children.
The spleen does not filter lymph - it is not connected to the lymphatic system in the same way as lymph nodes. It is a lymphoid tissue, however, with a role in the production of white blood cells that form part of the immune system. The spleen's other major role is filtering the blood to remove microbes and old and damaged red blood cells and platelets.
The thymus gland is a lymphatic organ and an endocrine gland. This means that it secretes hormones, as well as being crucial in the production and maturation of immune cells. The thymus gland is active in developing the immune system from before birth and through childhood.
The bone marrow is not lymphatic tissue in the same sense as the tissues above, but it can be considered part of the lymphatic system because it is responsible for maturation of the B cell lymphocytes of the immune system. For completeness, it should be noted that the liver of a fetus is regarded as part of the lymphatic system due to its involvement in lymphocyte development.
Lymphatic system function: fluid homeostasis
One of the major roles of the lymph vessels is to maintain fluid balance, returning excess fluid and proteins from the tissues that cannot be returned via venules and veins.
The fluid is found in tissue spaces and cavities, in the tiny spaces surrounding cells (the interstitial spaces) reached by the smallest blood and lymph capillaries.
Some 90% of the plasma arriving at tissues from the arterial blood capillaries is returned by the venous capillaries and back along veins; the remaining 10% is drained back by the lymphatics.10
The amount of lymph returned each day amounts to a total of around 2-3 liters, and this fluid includes proteins that are too large to be transported via the blood vessels.
While not as immediately fatal as failure of blood circulation, loss of the lymphatic system would lead to death within a day. Without the lymphatic system and draining excess fluid our tissues would swell greatly, causing lost blood volume and pressure.
Lymphatic system function: absorption
The lymphatic system takes up fats from the small intestine through the activity of tiny lacteals.
Almost all 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 protruding structures (the finger-like villi) 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, which contains lymph and emulsified fats, or free fatty acids, delivers nutrients indirectly when it reaches the venous blood circulation. Blood capillaries take up other nutrients directly.
Lymphatic system function: the immune system
The lymphatic system forms a major part of our immune response to the continual exposure to micro-organisms. Some such organisms are potentially harmful and even fatal as there are some infections that our immune system is not equipped to deal with.6-9
The lymphatic system is not the first line of defense against infectious agents such as bacteria and viruses however, as these organisms must first content with:
- Physical barriers such as the skin
- Toxic barriers such as the acidic contents of the stomach
- Competition posed by the so-called friendly organisms in the body that are normally not harmful and are often beneficial.
Despite the effectiveness of these barriers, numerous pathogens do successfully invade the body, whereupon they can cause infection if they are not promptly dealt with by the immune system. A number of different immune cells and special molecules work together to fight off these pathogens, with complex cascades of immune activity designed to recognize and destroy the foreign material.
The lymphatic system plays a key role in this activity as lymph fluid containing foreign organisms is drained from the tissues and is presented to immune system cells. These cells can then form antibodies to pathogens or produce antibodies from memory if they have previously encountered the specific pathogen.
A radioactive injection that finds its way to the lymph nodes, making it easier for doctors to identify those that may be draining certain cancerous tumors.
News in May 2015 about clues into cancer cell metastasis.
Research published in the journal Biomaterials in November 2015 found a nanoparticle way to harness immune cells against cancer cells in lymph nodes.
Collections of lymph nodes are concentrated in the neck, armpits and groin. The presence of lymph nodes becomes obvious on one or both sides of the neck when we have so-called swollen glands.
Lymph is filtered by the lymph nodes and other lymphoid organs. The lymph nodes provide an environment for immune cells known as lymphocytes - a type of white blood cell - to first encounter pathogens, communicate with each other and set off a specific response to these antigens.
Activated lymphocytes then get trafficked further up the lymphatic system so that they can reach the bloodstream, equipped to disseminate the immune response right around the body via 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" - highly specific and long-lasting responses to particular pathogens.
Adaptive immunity is much more sophisticated than the "innate immune response" that we also employ and that invertebrate animals rely on alone to mount a non-specific response to pathogens - our innate immunity includes "phagocytic cells," which ingest and destroy microbes.
Lymphatic system diseases
The lymphatic system can become dysfunctional if nodes, ducts, vessels or lymph tissues become blocked, infected, inflamed or cancerous. This can lead to a combination of two or three of the following broad features of lymphatic disorders:1
- When a lymphatic disorder involves obstruction, lymph fluid builds up in the tissues, a condition known as lymphedema (also spelled lymphoedema)
- Infection can lead to enlargement of the lymph nodes (see below)
- Cancer - the least common but most serious lymphatic disease, lymphoma is usually secondary and arises when cancer spreads from a primary tumor (such as in the breast) to nearby or regional lymph nodes. It is rare for a cancer to start in the lymphatic system itself (a primary cancer).
What are swollen lymph nodes?
Swollen glands - for example, in the neck during a throat infection - are enlarged lymph nodes. Lymph nodes can swell for two common reasons:
- Reaction to an infection (reactive lymphadenopathy) - when lymph nodes react to foreign material presented to immune cells through the lymph drained from infected tissue
- Direct infection of the lymph nodes themselves leading to their inflammation (lymphadenitis) - usually associated with certain infections that need prompt antibiotic treatment.
As a general guide, there is usually no need for people who are otherwise healthy to go to the doctor if common, self-limiting problems such as a cold or influenza can explain swollen glands that settle down.
However, it is best to seek medical advice if in doubt or if lymph nodes stay swollen for longer than a week or two. Medical advice should also be sought if a swollen lymph node feels hard or fixed in place, or if swelling is accompanied by fever, night sweats or unexplained weight loss.
Swollen lymph nodes can be symptomatic of numerous possible conditions. Glandular fever is one cause of longer-lasting swelling - this viral infection is also known as infectious mononucleosis, or mono.
Children are more prone to swollen lymph nodes because their immune systems are developing responses to infectious microbes.
Tonsillitis, for example, is more common in children. This condition occurs when the lymph nodes at the back of the mouth are fighting infection - usually viral, but less commonly a bacterial infection. Pharyngitis is one type of bacterial infection, commonly called "strep throat," caused by group A streptococcus bacteria.