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Immunity and infection prevention

The lymphatic system

Besides the blood vessels, the body has another network of fine tubes: the lymphatic system. It returns extra fluid from the tissues to the blood, carries fats from the gut and filters germs in the lymph nodes. This topic explains how the lymphatic system works, why lymph nodes swell and how this relates to leukemia.

Last updated: ~6 min read
A fine network of glowing turquoise vessels spread along a translucent human body on a dark navy background, with small shining nodes along it.
Figure 1.Concept: lymphatic vessels form a fine network that spans the whole body.

01Short answer

The lymphatic system is made up of a clear fluid called lymph, the lymphatic vessels that carry it, the lymph nodes and the lymphoid organs (spleen, thymus, tonsils). It has three main jobs: returning extra fluid that collects in the tissues to the blood and preventing swelling; carrying fats and fat-soluble vitamins from the gut; and filtering germs and organising the immune response. Unlike the circulation, the lymphatic system has no pump like the heart: lymph moves thanks to muscle movement and breathing. Lymph nodes cluster in the neck, armpits and groin and in the chest and abdomen. Nodes most often swell during an infection and go down on their own. In some leukemias, especially CLL and ALL, nodes can enlarge because of leukemia cells. Nodes that keep growing, feel hard or do not go away should be shown to a doctor.

02Simple explanation

Fluid seeps out of blood vessels into the tissues so that cells receive food and oxygen. Most of this fluid returns to the blood, but some stays in the tissues. Lymphatic vessels collect this extra fluid, and from then on it is called lymph.

The lymphatic vessels join up and get bigger, and finally empty into large veins near the neck. This is how the fluid gets back into the blood. If this system did not work, fluid would build up in the tissues and cause swelling.

On its way, lymph passes through lymph nodes. Nodes look like small beans and work as filters: the lymphocytes and macrophages inside them trap germs and foreign particles.

The lymphatic system has no pump. Lymph is moved by muscles contracting, breathing movements and the vessel walls themselves. That is why walking and gentle movement help lymph flow.

The link with leukemia: lymphocytes are the main cells of the lymphatic system. In lymphoid leukemias (CLL, ALL), leukemia cells can collect in the lymph nodes and make them bigger. In lymphoma, by contrast, the disease mainly starts in the lymph nodes themselves.

03Scientific explanation

Fluid balance: according to the NCI’s SEER training materials, about 90% of the fluid that leaves the capillaries returns to the blood, and the remaining 10% becomes part of the tissue fluid; lymph capillaries collect this extra fluid and proteins and return them to the venous blood. When this process fails, blood volume falls and swelling (oedema) develops in the tissues.

Carrying fats: according to SEER, the villi of the small intestine contain special lymph capillaries called lacteals. Most nutrients are absorbed into the blood capillaries, but fats and fat-soluble vitamins are carried by the lacteals; this milky, fat-rich lymph is called chyle.

Structure: according to SEER, lymph capillaries are found throughout the body except in the bone marrow, the central nervous system and tissues without blood vessels. Lymphatic vessels have valves that prevent backflow; lymph returns to the blood through the right lymphatic duct and the thoracic duct. Lymph nodes are usually smaller than 2.5 cm and cluster in the neck, armpits and groin.

Lymph nodes and leukemia: according to the NCI and the American Cancer Society, enlarged lymph nodes are common in CLL and are part of the Rai and Binet staging systems. According to the American Cancer Society, ALL can also spread to lymph nodes near the surface of the body, causing lumps that can be felt under the skin. The medical term for enlarged lymph nodes is lymphadenopathy.

04What happens in the body?

When should you see a doctor?

According to the NHS and MedlinePlus, swollen nodes are most often linked to an infection and go down within 1–2 weeks. See a doctor if nodes keep growing or do not go away, feel hard and fixed, or come with night sweats, fever or unexplained weight loss. If you get a fever during treatment, call urgently.

How does a lymph node work?

Lymph enters a node through several incoming vessels, passes through spaces inside it and leaves through one outgoing vessel. Lymphocytes and macrophages are packed tightly inside the node. When a germ arrives, these cells multiply, the node enlarges and is sometimes tender. Once the infection has passed, the node gradually returns to its usual size.

A translucent bean-shaped lymph node on a dark navy background, packed with glowing lymphocytes and joined by fine light-filled vessels.
Figure 2.A lymph node works like a filter: lymphocytes trap germs.

Where does lymph flow?

Lymph flows upwards from the legs, arms and organs towards the neck. Lymph from the upper right part of the body collects in the right lymphatic duct, and lymph from the rest of the body in the thoracic duct. Both ducts empty into large veins at the base of the neck, and the lymph mixes with the blood again.

The point where a thin translucent lymphatic vessel joins a wider vein glowing reddish on a dark navy background, with soft light where they meet.
Figure 3.Near the neck, lymph mixes with the blood again.

Lymphoedema

When lymph vessels or nodes are damaged, fluid builds up in the tissues, causing long-lasting swelling of an arm or leg. According to the NCI, this happens mainly after surgical removal of lymph nodes or radiation therapy; it is rare in leukemia. Tell your team about any new swelling.

How can you help your lymphatic system?

Because the lymphatic system has no pump, movement matters: walking, gentle exercise and deep breathing all help lymph flow. Take good care of cuts and scratches on the skin, because an infection can make nearby nodes swell. Agree on how much physical activity is right for you with your team, based on your blood counts.

05The path of lymph

From the tissues to the blood (simplified).

Tissue fluid — Some fluid from the blood stays in the tissues.1Tissue fluidSome fluid from the blood stays in the tissues.
Lymphatic vessel — The fluid is collected and becomes lymph.2Lymphatic vesselThe fluid is collected and becomes lymph.
Lymph node — Germs are filtered and an immune response starts.3Lymph nodeGerms are filtered and an immune response starts.
Vein — Lymph returns to the blood near the neck.4VeinLymph returns to the blood near the neck.
Fluid balancePrevents swelling.
FilterNodes trap germs.
DefenceLymphocytes meet the germ.

06Swollen lymph nodes: common causes and warning signs

FeatureUsually a common causeShould be shown to a doctor
DurationGo down within 1–2 weeksDo not go away or keep growing
How they feelSoft, tender, movableHard, painless, fixed
Other signsA cold, sore throat, tooth or skin infectionNight sweats, unexplained weight loss, lasting fever
LocationOne area near an infectionSeveral areas at once
Blood testNormal or fits an infectionUnexplained changes in blood counts

* This table is only a general guide and cannot be used to make a diagnosis. Show your doctor any node that worries you.

07Key terms

Lymph
A clear fluid collected from the tissues, formed from plasma.
Lymphatic system
The system made of lymph, lymphatic vessels, nodes and lymphoid organs.
Lymphatic vessel
A thin tube that carries lymph from the tissues towards the blood.
Lymph node
A small bean-shaped organ that filters lymph.
Lymphadenopathy
Enlargement or disease of the lymph nodes.
Thoracic duct
The main vessel returning lymph from most of the body to the blood.
Lacteal
A special lymph capillary in the gut that absorbs fats.
Oedema
Swelling caused by fluid building up in the tissues.
Lymphoedema
Long-lasting swelling caused by blocked lymph flow.
Lymphocyte
The main immune cell of the lymphatic system.

08Key points

  • The lymphatic system returns extra fluid from the tissues to the blood.
  • It carries fats and fat-soluble vitamins from the gut.
  • Lymph nodes are filters that trap germs.
  • The lymphatic system has no pump; movement helps lymph flow.
  • Nodes most often swell during infections and go down on their own.
  • In CLL and ALL, nodes can enlarge because of leukemia cells.
  • Show a doctor any node that keeps growing, feels hard or does not go away.
  • A fever during treatment is an emergency.

09Frequently asked questions

How is the lymphatic system different from the circulation?

The circulation is a closed loop driven by the heart. The lymphatic system is a one-way route: lymph flows from the tissues towards the blood, and it has no pump. The two systems join near the neck.

Are lymph nodes always enlarged in leukemia?

No. They often enlarge in CLL and ALL, but less often in AML and CML. The size of the nodes is one of the pieces of information that help a doctor assess the type and stage of leukemia.

Are leukemia and lymphoma the same thing?

No, but they are related. Leukemia mainly starts in the bone marrow and blood, while lymphoma mainly starts in the lymph nodes and lymphoid tissue.

Is massage or “lymphatic drainage” helpful in leukemia?

It does not treat leukemia. Gentle massage can sometimes bring comfort, but with low platelets there is a risk of bruising, and with broken skin a risk of infection. Check any procedure with your team first.

I can feel a small lump in my neck. Should I worry?

Small, soft, movable nodes can be felt in healthy people too and are often left over from past infections. If a node grows, hardens or comes with other signs, show it to a doctor.

Do nodes shrink after treatment?

Nodes enlarged by leukemia cells usually shrink with effective treatment. Their size is also tracked to judge the response to treatment.

10Questions for your doctor

Take these questions to your appointment. The ones you save are collected on the “My questions” page and stay on this device only.

  1. Is the swelling of my lymph nodes related to the leukemia?

  2. How will the size of my lymph nodes be monitored during treatment?

  3. What should I do if my arms or legs swell?

  4. Which lymph node changes should I report?

My questions

12Sources

  1. National Cancer Institute (NCI), SEER Training. Introduction to the Lymphatic System
  2. National Cancer Institute (NCI), SEER Training. Components of the Lymphatic System
  3. National Cancer Institute (NCI), SEER Training. Lymph Nodes
  4. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: lymphatic system
  5. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: lymph
  6. National Cancer Institute (NCI). NCI Dictionary of Cancer Terms: lymphadenopathy
  7. Cancer Research UK. The lymphatic system and cancer
  8. MedlinePlus (U.S. National Library of Medicine). Lymphatic Diseases
  9. MedlinePlus (U.S. National Library of Medicine). Swollen lymph nodes
  10. NHS (UK). Swollen glands
  11. National Cancer Institute (NCI). Lymphedema and Cancer
  12. National Cancer Institute (NCI). Chronic Lymphocytic Leukemia Treatment (PDQ®) – Patient Version
  13. American Cancer Society. Chronic Lymphocytic Leukemia Stages
  14. American Cancer Society. Signs and Symptoms of Acute Lymphocytic Leukemia
Medical disclaimerThis information is for educational purposes only and does not replace medical advice.
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