Lymph nodes are part of the body’s immune system. They act as filters, removing potentially harmful substances from the body. A swollen lymph node in the armpit may be a sign of an infection or injury.

The possible causes of lymph node swelling range in severity from common infections that typically resolve on their own to more serious conditions, such as lymphoma.

In this article, we look at why lymph nodes swell, the most common causes of this symptom, and when to consult a doctor.

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When a person has an infection or injury, the lymph nodes may swell as they start to filter unwanted cells from the lymph.

Lymph is a watery fluid that carries oxygen to the cells and transports waste products away from them. It also contains white blood cells, which help fight infections.

As the lymph nodes begin to work harder to remove waste, they can enlarge. This enlargement is more common in certain areas of the body, including the neck, armpits, and groin.

A swollen lymph node may be painful and tender to the touch. In some cases, it will be visibly enlarged under the skin, but in others, it will be smaller or deeper in the body and only apparent when touching the area.

Many viruses can cause swollen lymph nodes. These include:

Infections with these viruses usually produce other visible symptoms, such as a rash.

However, other viral conditions can cause swollen lymph nodes with no other visible symptoms. These include:

Influenza (flu)

The flu is a respiratory infection that can also cause the lymph nodes to swell. The symptoms of the flu are similar to those of other respiratory viruses, but they tend to be more severe. They also often develop suddenly rather than gradually.

Other symptoms of the flu include:

Vomiting and diarrhea can also occur, but these symptoms are more common in children.

While a person has the flu, they should stay at home and rest, avoiding contact with others. Most people recover from the flu without treatment, but it can sometimes cause complications.

People who are most at risk of complications include:

  • young children
  • adults over 65 years of age
  • pregnant people
  • people with underlying health conditions

People in these groups may need antiviral medication to prevent severe symptoms. Getting a flu vaccine each year is the best way to avoid getting the flu.

Infectious mononucleosis

Infectious mononucleosis, also known as mono or glandular fever, is the result of a viral infection. It can cause lymph nodes in the neck and armpits to swell. Mono also causes symptoms such as:

  • extreme fatigue
  • fever
  • swelling in the liver, spleen, or both
  • sore throat
  • body aches
  • headaches

Mono will eventually go away on its own. Most people recover in 2–4 weeks, but some experience symptoms for longer. Resting, drinking fluids, and taking over-the-counter (OTC) pain relievers can help during recovery.

Bacterial infections can also cause the lymph nodes to swell. Some examples of infections that could affect the nodes in the armpit include:

Cellulitis

Cellulitis is a skin infection. It occurs when bacteria penetrate the skin and infect the deeper layers, potentially as a result of an injury that led to an area of broken skin.

Cellulitis may cause nearby lymph nodes to swell. For example, an infection in the arm may cause the lymph nodes in the armpit to enlarge. Common symptoms of cellulitis at the infection site include:

  • pain and swelling
  • skin sores
  • skin that is warm to the touch
  • redness, which may be less apparent in people with dark skin tones
  • hardening of the skin
  • fluid collection under the skin

Additional symptoms of cellulitis may include:

  • fever or chills
  • body aches
  • muscle and joint pain
  • vomiting and nausea
  • fatigue

Doctors treat cellulitis with antibiotics. A person may need to stay in the hospital if the infection is severe or they require IV antibiotics, which a doctor administers directly into a vein.

Lyme disease

Lyme disease spreads via the saliva of certain species of tick, which are small insects that can bite humans. One of the early symptoms of Lyme disease is swollen lymph nodes, which may appear 3–30 days after the tick bite occurred.

Other early symptoms include:

  • a circular rash resembling a bull’s-eye at the site of the bite
  • fever
  • chills
  • joint or muscle aches
  • fatigue
  • headaches

A doctor will typically prescribe antibiotics to treat Lyme disease. Anyone who suspects that they have this condition should seek medical attention promptly.

Other bacterial infections that can cause swollen lymph nodes include:

However, these infections typically affect the lymph nodes in other areas of the body, such as the neck or groin. They are less likely to cause swelling in the armpits.

Bacteria and viruses are not always responsible for swollen lymph nodes in the armpit. Other possible causes include:

Rheumatoid arthritis

Rheumatoid arthritis (RA) is one of several autoimmune conditions that can cause swollen lymph nodes.

RA occurs when the body’s immune system mistakenly attacks the lining of the joints, causing stiffness, pain, and warmth.

A 2019 review article states that RA affects the lymph nodes, reducing their capacity to drain fluid from nearby inflamed joints. This impairment may lead to local lymph node enlargement.

Doctors treat RA with medications that reduce inflammation and relieve pain. Physical therapy may also help. In some cases, a doctor may recommend surgery to replace or repair affected joints.

Cancer

In some cases, swollen lymph nodes are a symptom of cancer.

Cancer that begins in the lymphatic system is known as lymphoma. There are several types of lymphoma, including:

  • Hodgkin lymphoma
  • non-Hodgkin lymphoma
  • non-Hodgkin lymphoma in children
  • Waldenström macroglobulinemia
  • lymphoma of the skin

In addition to swollen lymph nodes, the symptoms of lymphoma can include:

  • unintentional weight loss
  • feeling tired
  • fever
  • night sweats

Other types of cancer that have spread to the lymph nodes, such as breast cancer, can also cause swelling in these parts of the body.

The type and stage of the cancer, as well as a person’s age and overall health, will affect what treatment doctors recommend.

However, it is worth remembering that there are many causes of swollen lymph nodes that are not related to cancer.

A doctor can determine the cause of swollen lymph nodes in the armpit and recommend the best treatment. They may ask about the person’s symptoms, review their medical history, and perform a physical examination.

In some cases, a doctor may also carry out diagnostic tests, such as blood tests, a biopsy, or medical imaging.

In most cases, the swelling in lymph nodes under the armpit will resolve within 1–2 weeks.

If the swelling lasts for longer or worsens over time, a person should speak with a doctor.

Swollen lymph nodes can be painful. While a person receives medical treatment, they can also try certain techniques at home to ease any tenderness.

For instance, a person can apply a warm compress to reduce pain. They can run warm or hot water over a washcloth and wring it mostly dry before placing it on the swollen lymph node.

People can also take OTC pain medications, such as acetaminophen or ibuprofen, to relieve pain. A person should talk with their doctor if they are not sure what medications are best for them.

Anyone with swollen lymph nodes in their armpit should talk with a doctor. Swollen lymph nodes have many potential causes, and a doctor can rule out possibilities that require prompt treatment, such as Lyme disease.

Although swollen lymph nodes often result from an infection, it is important for a person to schedule an appointment if:

  • the swelling continues for more than 2 weeks or worsens after this time
  • the lump feels hard or does not move when a person touches it
  • there is swelling in lymph nodes in more than one area — for example, in both the neck and armpits
  • the swollen lymph nodes are not painful
  • there are other symptoms, such as fever, night sweats, or unexplained weight loss

A person should also consult their doctor about swollen lymph nodes if they have previously had cancer treatment.

Swollen lymph nodes in the armpit can be a sign of common viral infections, such as the flu or mono. They can also occur as a result of a bacterial infection or RA. In some cases, swollen lymph nodes are a symptom of cancer.

Warm compresses and OTC pain medication can ease any pain or tenderness. However, a person should talk with a doctor if they have swollen lymph nodes with no clear cause.