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.
On the next page, we look in more detail at how the lymphatic system drains excess fluids, absorbs fats from the gut and develops immune responses. Selected disorders are also introduced and "swollen glands" are explained.