A swollen armpit is usually a sign that your body is fighting an infection nearby, though it can also come from skin conditions, recent vaccinations, or less commonly, something more serious. The swelling is most often a lymph node doing its job. Lymph nodes are small, bean-shaped filters packed throughout your body, and your armpit contains a large cluster of them. When they detect bacteria, viruses, or other threats draining from your arm, chest, or breast tissue, they swell up as they ramp up their immune response.
Infections Are the Most Common Cause
Infections or injuries of the upper extremities are the most frequent reason for armpit lymph node swelling. A cut on your hand, a skin infection on your arm, or even a bad case of the flu can trigger it. The nodes swell because they’re filtering out the invading organisms and producing immune cells to fight them. Once the infection clears, the swelling typically goes down on its own within a couple of weeks.
Cat scratch disease is one of the more specific infections linked to armpit swelling. If a cat scratches or bites your hand or arm, bacteria can travel through your lymphatic system and settle in the nearest lymph nodes. More than 65% of cat scratch cases involve the armpit or neck nodes. The swelling usually appears one to three weeks after the scratch and resolves within two to four months, though it can occasionally linger up to a year. Over half of people with cat scratch disease have just a single swollen node with no other symptoms.
Skin Conditions That Cause Armpit Lumps
Not every armpit lump is a lymph node. The armpit is a warm, moist environment where skin rubs together, making it a common spot for several skin-related problems.
Boils (also called abscesses) develop when bacteria infect a hair follicle. They start as a red, tender bump that fills with pus over several days. Most boils eventually drain on their own or with warm compresses, but larger ones sometimes need to be drained by a doctor.
Hidradenitis suppurativa is a chronic condition that causes small, painful lumps under the skin in areas where skin rubs together. It tends to start after puberty and before age 40. The key difference from a simple boil is that these lumps recur, heal slowly, and can eventually form tunnels under the skin that drain blood or pus. If you keep getting painful bumps in your armpits (or groin, buttocks, or under the breasts) that take weeks or months to resolve, this condition is worth discussing with a doctor. It’s often misdiagnosed as recurring boils for years before getting the right treatment.
Vaccine-Related Swelling
If you recently got a vaccine in your arm, armpit swelling on that same side is a well-documented and harmless reaction. It was especially common with COVID-19 mRNA vaccines. In Moderna’s clinical trials, 16% of women aged 18 to 64 developed armpit swelling after the second dose. Pfizer’s data showed the swelling typically appeared two to four days after vaccination and lasted an average of 10 days. Flu shots and other arm-injected vaccines can cause the same thing, just less frequently. No treatment is needed; it resolves on its own.
When Swelling Could Signal Something Serious
In a small but important number of cases, a swollen armpit node points to cancer. Breast cancer, lymphoma, melanoma, and lung cancer can all cause armpit lymph node enlargement. When doctors evaluate an enlarged armpit node and find no obvious infection or injury to explain it, they consider malignancy more seriously. In studies of patients with unexplained enlarged armpit nodes on one side, roughly one in four turned out to have a malignant cause.
Cancerous lumps tend to be hard, painless, fixed in place (they don’t move when you push them), and they grow over time rather than shrinking. But size and feel alone aren’t reliable predictors. Ultrasound imaging is much better at distinguishing normal from abnormal nodes by looking at their internal structure: a healthy node has a thin outer layer and a bright fatty center, while a suspicious node loses that normal architecture, becomes round, and thickens unevenly.
Silicone breast implants can also cause armpit swelling. Tiny silicone particles from implant leakage trigger an inflammatory reaction in nearby lymph nodes. This isn’t cancer, but it does need medical evaluation to confirm the cause.
Red Flags Worth Paying Attention To
Most armpit swelling resolves within two weeks as the underlying cause clears up. You should get it checked if the lump:
- Persists beyond two weeks without improving
- Feels hard and doesn’t move under the skin
- Keeps getting bigger
- Comes with a fever, unexplained weight loss, or drenching night sweats
- Appears suddenly with no obvious cause like a recent infection, injury, or vaccination
What to Expect at a Doctor’s Visit
A doctor will typically start by feeling the lump and asking about recent infections, injuries, cat exposure, vaccinations, and other symptoms. If the cause isn’t obvious from that alone, an ultrasound is usually the first imaging test. It’s painless and gives detailed information about the node’s shape, internal structure, and blood flow. Normal nodes appear oval with a thin outer layer no thicker than 3 millimeters and a bright fatty center. Nodes that have lost that normal pattern, appear round, or have uneven thickening get flagged for closer evaluation.
If the ultrasound looks suspicious, the next step is a needle biopsy. A thin needle is inserted into the node (guided by ultrasound) to collect a small tissue sample. The procedure takes minutes and is done with local numbing. Results typically come back within a few days and can confirm or rule out infection, lymphoma, or metastatic cancer.
Simple Home Care for Mild Swelling
When the swelling is mild and likely related to a minor infection or recent vaccination, warm compresses can help ease discomfort. Soak a washcloth in hot water, wring it out, and hold it against the swollen area for 10 to 15 minutes a few times a day. Over-the-counter pain relievers like ibuprofen or acetaminophen can reduce both pain and inflammation. If a bacterial infection is the cause, your doctor may prescribe antibiotics, which typically bring the swelling down within a week or two. Viral infections won’t respond to antibiotics and simply need time to run their course.