Why Do Lymph Nodes in the Armpit Swell?

Why do lymph nodes in the armpit swell?

A lump noticed in the armpit often first raises fears of cancer. Yet the most common cause of swollen lymph nodes in the armpit is the immune system's normal response to an infection in the arm, hand or chest wall, or to a recent vaccination. The likelihood of cancer among people presenting to primary care with swollen lymph nodes is approximately 1 percent. Nevertheless, certain features call for closer assessment. Drawing on current sources, this article explains why armpit lymph nodes swell, which situations are minor, which are warning signs, and how doctors assess them.

What are lymph nodes, and what do those in the armpit do?

Lymph nodes, also called lymph glands, are small, bean-shaped organs that filter lymph fluid and house immune cells. Although the number varies between regions of the body and between individuals, there are between 400 and 800 lymph nodes in the body. As lymph collected from tissues passes through these nodes, microbes, foreign substances and damaged cells are trapped, and the lymphocytes (B and T cells) within the node become activated. We explain the general structure of lymph nodes and their locations in the body in our article what are lymph nodes in greater detail.

Each armpit contains approximately 20 to 40 lymph nodes. These nodes collect lymph from the arm and hand, breast and chest wall. Swollen armpit nodes therefore often indicate a problem in the arm, hand, breast or chest wall on the same side. A healthy lymph node is usually too small to feel; in thin people, small, soft and mobile nodes may also be felt normally.

A node that traps a microbe or foreign substance in a particular region enlarges as its immune cells multiply; this is called reactive lymphadenopathy. If the node itself becomes infected and inflamed, a painful, red and warm swelling develops; this condition is called lymphadenitis. Enlargement caused by cancer cells results either from a cancer of the lymphatic system itself (lymphoma) or from cancer spreading to the node from another organ (metastasis).

Whether the swelling is on one side or both sides is an important clue. One-sided swelling usually points to a local cause on the same side, whereas swelling on both sides or in several regions of the body is more suggestive of a disease affecting the body as a whole.

Main causes of swollen armpit lymph nodes

The causes of swollen armpit lymph nodes can be grouped under five main headings: infections, vaccinations, immune and inflammatory diseases, foreign substances (such as breast implants) and cancer. The table below summarizes the most common causes and the clues that help distinguish them.

1. Infections: the most common cause

The most common cause of swollen armpit nodes is an infection of the arm, hand, breast or armpit skin on the same side. An infected ingrown fingernail, a cut on the arm, an insect bite, folliculitis following shaving or waxing, or a breast infection (mastitis) can enlarge armpit nodes. These nodes are typically painful, soft and mobile; they shrink as the underlying infection resolves.

Hidradenitis suppurativa, a chronic skin disease affecting hair follicles in skin folds such as the armpits and groin, causes recurrent painful swellings and draining tracts. People often mistake these swellings for lymph nodes. A dermatologist diagnoses and treats the condition.

Cat scratch disease is an infection caused by the bacterium, Bartonella henselae. Painful lymph node swelling develops 1-3 weeks after a cat scratch, bite or a cat licking an open wound, usually in the region closest to the scratch, such as the armpit. According to the United States Centers for Disease Control and Prevention (CDC), most cases resolve without antibiotics; people with a suppressed immune system require antibiotic treatment. Contact with kittens is particularly relevant to this disease.

Infections that affect the whole body (systemic infections) may enlarge nodes in the neck and groin as well as the armpit. This group includes viral infections such as infectious mononucleosis (the kissing disease), cytomegalovirus, HIV and rubella, as well as toxoplasmosis. Other symptoms, such as fever, a sore throat and fatigue, are usually also present.

Tuberculosis, brucellosis and tularemia are additional infections to bear in mind in Türkiye (details appear in the Türkiye box below). In Lyme disease, 3-30 days after a tick bite, an expanding rash (erythema migrans) occurs in approximately 70-80% of patients; swollen lymph nodes may also develop without a rash. Sexually transmitted infections such as chlamydia and syphilis mostly enlarge groin nodes; they are not typical causes of armpit swelling.

2. Vaccinations

Vaccines are designed to stimulate the immune system. As a natural result of this stimulation, armpit nodes on the side of the vaccinated arm may temporarily swell and become tender. This became widely recognized with COVID-19 vaccines, but can also occur after influenza, pneumonia (pneumococcal), HPV, chickenpox-shingles and other vaccinations.

In the clinical trial of Moderna's mRNA COVID-19 vaccine, the rate of armpit swelling or tenderness after the second dose among people aged 18-64 was 16.0% in the vaccine group, versus 4.3% in the placebo group (8.4% versus 2.5% in those over 65; 21.0% versus 5.0% among those aged 12-17). In other words, vaccination made this complaint in people aged 18-64 approximately 3.7 times as common; for every 100 vaccinated people, the absolute difference was approximately 12 people. These swellings usually resolve on their own within a few days to a few weeks.

Nodes enlarged after vaccination may also appear as a "suspicious finding" on imaging such as mammography, ultrasound, MRI and PET-CT. We discuss the PET-CT aspect in our article false-positive lymph nodes on PET scans after COVID-19 vaccination. For practical recommendations from breast imaging specialists, see the "Vaccination and mammography" section below.

3. Immune diseases, medicines and foreign substances

Diseases such as rheumatoid arthritis, lupus (systemic lupus erythematosus), Sjögren syndrome and sarcoidosis may cause lymph nodes to enlarge in several regions because of persistent stimulation of the immune system. Some medicines, such as certain epilepsy medicines and the gout medicine allopurinol, may also rarely cause widespread lymph node enlargement.

Silicone breast implants may cause armpit swelling when silicone migrates to lymph nodes after an implant leaks or ruptures. New armpit swelling in a person with an implant should be assessed on the implant side. The rare condition Castleman disease also causes enlarged lymph nodes; it is not cancer, but may require treatment.

4. Cancer

Cancers that cause armpit lymph nodes to enlarge fall into two groups:

• Lymphoma (lymphatic cancer): This is cancer of the lymphatic system itself and has two main groups: Hodgkin lymphoma and non-Hodgkin lymphoma. It typically presents with painless, rubbery nodes that grow over a period of weeks; fever, night sweats and weight loss (B symptoms) may accompany them. For symptoms and treatment, see our article what is lymphoma.
• Spread of another cancer (metastasis): The cancer that most commonly spreads to armpit nodes is breast cancer. Another cancer affecting the skin of the arm, shoulder, chest or back, melanoma, can also spread to these nodes. Less commonly, cancers of the lung, thyroid, stomach, colon, pancreas, ovaries and kidneys may also reach armpit nodes.

Nodes enlarged because of cancer are usually hard, painless, progressively growing and fixed to surrounding tissues or to each other (immobile). However, these features alone do not establish a diagnosis: some lymphomas may be soft, while some inflamed nodes may also become hard.

Could an armpit lump be cancer?

It could, but the likelihood is low. According to a 2016 review by the American Academy of Family Physicians (AAFP), the cancer detection rate among people presenting to primary care with unexplained swollen lymph nodes is approximately 1.1%. This rate rises markedly with age: in people aged 40 and over, it is 4%, whereas the rate in those under 40 is 0.4%. In other words, among those under 40 with swollen lymph nodes, cancer is found in approximately one in every 250 people; among those over 40, it is found in one in every 25.

The location of the swelling also matters. The AAFP review reports a cancer likelihood of 34-50% for enlarged lymph nodes above the collarbone (supraclavicular nodes), so swelling in this region should always be assessed. A node above the left collarbone may be associated with the spread of cancer from abdominal organs; we describe its history in our article what is Virchow's node.

Warning signs: when should you see a doctor without waiting?

• Size and course: A node larger than 1 cm that does not shrink or continues to grow. Nodes larger than 1 cm in the armpit are generally considered abnormal.
• Consistency: A hard, painless node fixed to surrounding tissues or other nodes (unable to move).
• Location: Swelling above the collarbone, or nodes enlarging simultaneously in several regions (neck, armpit, groin).
• General symptoms: Unexplained fever, night sweats severe enough to require changing the sheets, unintentional weight loss or persistent itching.
• Breast changes: A lump in the breast on the same side, nipple retraction or discharge, or an orange-peel appearance of the skin.
• A changing skin mole: A mole on the arm, shoulder or trunk on the same side that changes color, shape or size.
• Risk factors: Being over 40, male sex, a previous cancer diagnosis or a suppressed immune system.

According to AAFP recommendations, one-sided swelling with an obvious cause (such as an infected wound on the hand or a vaccination a few days earlier) and no warning signs can be observed for approximately 4 weeks. If it does not shrink or grows during this period, a doctor should be consulted. Cleveland Clinic also recommends medical assessment for armpit swellings that do not resolve within two weeks or are hard, painful or growing.

How does a doctor assess it?

Assessment begins with a detailed history and examination. Your doctor will ask how long the swelling has been present, whether it is growing and whether it is painful; about recent vaccination, wounds, infection, cat contact, tick bites, raw milk or fresh cheese consumption, travel and new medicines; and about general symptoms (fever, sweating, weight loss). The examination assesses the node's size, consistency and mobility, other lymph node regions, the breasts, skin of the arm and hand, spleen and liver.

If considered necessary, the following tests may be requested:

• Blood tests: Complete blood count and peripheral blood smear, inflammatory markers (sedimentation rate, CRP), liver tests and, depending on the history, tests for infections such as mononucleosis, HIV, brucellosis, Toxoplasma and Bartonella.
• Ultrasound: This is the first-choice imaging method for assessing armpit nodes; it uses no radiation, shows the node's internal structure and can guide a biopsy if needed. Mammography is also added for women depending on age.
• Biopsy: An ultrasound-guided fine needle aspiration or core needle (tru-cut) biopsy is performed on a suspicious node; if needed, the entire node is surgically removed. If lymphoma is suspected, core needle biopsy or removal of the entire node is preferred to assess the arrangement of the cells. We explain biopsy methods in our article what is a biopsy. We also explain that biopsy does not spread cancer in this article.
• Advanced imaging: After cancer has been diagnosed, CT or PET-CT may be used to determine the extent of the disease. Results are interpreted carefully because infection, vaccination and inflammation can also produce uptake on PET-CT; see our article false-positive PET-CT results for details.

Normal and suspicious lymph nodes on ultrasound

A normal lymph node on ultrasound is oval; its outer layer (cortex) is thin, and at its center there is a bright-appearing fatty hilum. A suspicious node becomes rounder, its cortex thickens and its fatty hilum disappears. Loss of the fatty hilum is the most specific finding associated with cancer; however, inflamed nodes may have a similar appearance. A suspicious ultrasound finding is therefore a reason for biopsy, rather than a diagnosis.

Vaccination and mammography: should breast imaging be postponed?

In 2021, as COVID-19 vaccination became widespread, some organizations recommended performing screening mammography before vaccination or a few weeks afterward, out of concern that post-vaccination armpit swelling could lead to unnecessary additional tests. The Society of Breast Imaging (SBI) updated these recommendations in February 2022:

• Vaccination should not delay breast cancer screening.
• Enlarged armpit nodes on the vaccinated side, with no breast findings and an explanation consistent with vaccination, can in most cases be considered benign (BI-RADS 2) and may not require additional follow-up.
• If follow-up is considered necessary, reassessment is recommended after waiting at least 12 weeks following vaccination.
• Published studies report post-vaccination lymph node enlargement on imaging at rates ranging from 2.4% to 35%.

In practice: When attending for mammography or breast ultrasound, always tell the team whether you have been vaccinated in recent months, and state in which arm and on which date you were vaccinated. For people diagnosed with breast cancer or under follow-up, the imaging team may recommend vaccination in the opposite arm rather than on the cancer side. We compile screening recommendations for 2026 Breast Cancer Awareness Month in our article Pink October 2026.

Armpit lymph nodes and breast cancer

The first site to which breast cancer spreads is usually the armpit lymph nodes on the same side. Their status is therefore very important for staging and treatment planning. We explain the relationship between breast cancer and armpit nodes, sentinel lymph node biopsy and current approaches favoring less surgery in this article in detail.

Rarely, the spread of breast cancer may be detected only in an armpit node, without a palpable breast lump or a mass visible on mammography. This is called occult breast cancer and accounts for less than 1% of all breast cancers. Breast MRI is used to look for the source in the breast. You can read about breast cancer symptoms other than a lump in our article breast cancer symptoms other than a palpable lump.

It should be remembered that breast cancer also occurs in men. Armpit swelling accompanied by a hard lump behind the nipple in a man should always be assessed.

What can and cannot be done at home?

Antiperspirants and deodorants can cause folliculitis or skin irritation; however, there is no reliable evidence that these products cause breast cancer.

Frequently asked questions

Is every lump felt in the armpit a lymph node?

No. An armpit lump may also be folliculitis, a lipoma, a cyst, an abscess, hidradenitis suppurativa or breast tissue extending into the armpit. Examination and, if needed, ultrasound distinguish them.

Which is more dangerous: a painful or a painless lymph node?

Pain and tenderness usually suggest inflammation and are often due to a benign cause. Hard, painless nodes that continue to grow require closer assessment. However, pain alone does not rule out cancer, nor does the absence of pain mean cancer.

My armpit became swollen after vaccination. What should I do?

Tender swelling on the side of the vaccinated arm is common and usually resolves within a few days to a few weeks. See a doctor if it continues to grow, does not shrink within 4-6 weeks or other warning signs are present. Always mention the vaccination date and which arm was vaccinated during mammography or ultrasound.

Can deodorant or shaving make lymph nodes swell?

Folliculitis or skin irritation after shaving or waxing may cause mild, temporary enlargement of armpit nodes. There is no reliable evidence that deodorants or antiperspirants cause breast cancer.

How long is it safe to wait?

One-sided swelling with an obvious local cause and no warning signs can be observed for approximately 4 weeks. If there are warning signs, you are over 40 or you have previously been diagnosed with cancer, it is more appropriate to seek medical attention without waiting.

Which doctor should I see?

A family physician or internal medicine specialist can perform the initial assessment. If there are breast findings, general surgery or breast surgery is involved; radiology or general surgery performs biopsy of a suspicious node; and medical oncology or hematology becomes involved if cancer is diagnosed.

Is a biopsy essential?

No. Most swollen lymph nodes resolve as the cause is treated or on their own, without a biopsy. Biopsy is performed on nodes with warning signs, suspicious ultrasound findings or failure to shrink during the observation period.

Is armpit swelling dangerous in children?

Children's lymph nodes respond more readily to infections, and swelling is very common; the cause is usually infection or vaccination. Cat scratch disease is a common cause in children. Warning signs also apply to children, and a pediatrician should assess them.

Glossary

Lymphadenopathy

Abnormal lymph node size or consistency; commonly called "swollen lymph nodes".

Lymphadenitis

Inflammation of the lymph node itself; usually a painful, red and warm swelling.

Reactive lymph node

A benign lymph node that enlarges in response to infection or stimulation in a nearby region.

Axilla

The medical name for the armpit; an "axillary lymph node" is a lymph node in the armpit.

Supraclavicular

The region above the collarbone.

Hilum

The central area where blood vessels enter and leave a lymph node, appearing fatty and bright on ultrasound.

Cortex

The outer layer of a lymph node; thickening is one of the suspicious ultrasound findings.

BI-RADS

The standard system classifying breast imaging results from 0-6; 2 means a "benign finding".

Metastasis

Spread of cancer from the organ in which it started to another region, such as the lymph nodes.

Fine needle aspiration / core needle biopsy

Diagnostic methods in which cells or a small piece of tissue are collected from a lymph node using a needle.