When the body fights an infection or injury, lymph nodes sometimes swell. Doctors call this a reactive lymph node. Treatment can depend on the cause.

Lymph nodes house B and T lymphocytes, which are essential types of white blood cells. These cells help the body fight off infections from bacteria and viruses. Lymph nodes also carry lymphatic fluid and connect via lymphatic vessels. Together, the lymph nodes and lymphatic vessels make up the lymphatic system, which is an essential part of the immune system.

When something goes wrong in the body, the lymphatic system produces cells to fight bacteria, viruses, and other causes of illness. Lymph nodes swell in this process. Lymph nodes may also swell in response to an injury as they work to prevent or stop an infection. Sometimes, lymph nodes can swell because of cancer.

Lymph nodes usually swell near the site of the injury. For example, lymph nodes behind or under the ears may swell when a person has an ear infection, while lymph nodes in the neck might swell in response to strep throat.

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Lymph nodes often swell under the arms and in the neck.

The main symptom of a reactive lymph node is swelling. Sometimes, the area around the lymph node also swells. The body contains hundreds of lymph nodes, and they swell most often in the neck and around the face, along the groin, and under the arms.

When lymph nodes swell because of an infection or injury, the swollen lymph node is usually:

  • smaller than 1.5 centimeters (cm) in diameter
  • soft
  • movable

The swelling is usually only in a specific lymph node or group of lymph nodes. A person who has many swollen lymph nodes throughout the body may have a severe systemic infection or possibly certain types of cancer.

Reactive lymph nodes often accompany an infection or injury. A person might notice other symptoms of the infection, such as:

  • fever
  • pain or tenderness near an injury
  • symptoms of illness
  • signs of infection near an injury, such as redness or swelling
  • generally feeling sick

A swollen lymph node does not necessarily mean the infection is severe. Some people’s lymph nodes swell very easily, and a person has no other symptoms.

The most common reasons lymph nodes swell include:

  • infections, such as skin infections, ear infection, or sinus infections
  • exposure to allergens
  • injury or irritation to the skin, such as from recent orthodontic work
  • dental health issues, such as a cavity or tooth infection
  • exposure to bacteria or viruses
  • mononucleosis, a highly contagious virus that can cause many lymph nodes to swell
  • skin irritation, such as from acne or a rash

Sometimes, a person may have an immune disorder that causes lymph nodes to swell. Less common causes of swollen lymph nodes include:

  • metastatic cancer, which is cancer that has spread from its original location to the lymph nodes
  • lymphoma, which is cancer of the lymph nodes
  • sepsis, which is a type of systemic infection that can become life-threatening

When a person has cancer or a severe infection, they may have many swollen lymph nodes. Swelling in a single lymph node is less likely to be a sign of a serious issue.

The right treatment for swollen lymph nodes depends on the cause. When a person has no symptoms of infection, the swelling often goes away on its own.

When infections cause a reactive lymph node, a person may require antibiotics. More severe infections may require a person to stay in the hospital or receive antibiotics through an intravenous needle. People with weak immune systems may also need to stay in the hospital, even for relatively minor infections.

Treatments are available for many forms of cancer, particularly if a doctor diagnoses them early. Depending on the type of cancer a person has, a doctor may recommend removing the lymph nodes, chemotherapy, or radiation.

A person with swollen lymph nodes should not avoid the doctor because they fear cancer. Delaying treatment gives the cancer time to grow and get worse.

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A person should see a doctor if a fever develops alongside swollen lymph nodes.

Swollen lymph nodes can be alarming, especially if the cause of the swelling is unclear. In most cases, however, swollen lymph nodes are a sign that the body’s immune system is working well.

People who have HIV or AIDS, who take immune system-suppressing drugs, or whose doctors have told them they have a weak immune system, should call the doctor if their lymph nodes swell.

For most people, it is safe to wait for 1–2 weeks to see if symptoms go away. See a doctor within a day or two if any of the following occur:

  • swollen lymph nodes appear after an injury to the skin
  • a newborn or infant has swollen lymph nodes
  • a fever develops alongside swollen lymph nodes

If a person has no signs of an infection, a swollen lymph node might be a sign that the body has successfully fought off an infection. It is safe to wait for about 2 weeks to see if the swelling decreases.

If the swelling does not go away, or if the lymph node is hard or larger than 1.5 cm in diameter, see a doctor.

Lymph nodes swell for many reasons, most of which are relatively harmless. Nevertheless, it is important to recognize a swollen lymph node as the body’s sign that it might be fighting an infection.

A doctor can help with determining whether a person has a serious infection that warrants treatment, as well as assessing for other potential causes of reactive lymph nodes.

Sometimes lymph nodes remain swollen long after an infection has disappeared. As long as the lymph node does not change or become hard, this is not typically a sign of a problem. If a person notices that a lymph nodes changes, hardens, or grows very large, they should see a doctor.