A ball-like lump in your armpit is almost always a swollen lymph node, a cyst, or an inflamed hair follicle. The armpit is packed with lymph nodes, oil glands, and hair follicles, so it’s one of the most common places on the body for lumps to appear. Most are harmless and resolve on their own, but certain characteristics warrant a closer look.
Swollen Lymph Nodes: The Most Common Cause
Your armpits contain clusters of lymph nodes, small bean-shaped glands that filter germs and abnormal cells from your body. When they’re actively fighting something off, they swell up and become easy to feel under the skin. A reactive lymph node can range from pea-sized to kidney-bean-sized or larger, and it’s often tender or sore to the touch.
The most frequent triggers are ordinary infections: a cut or scratch on your hand or arm, a skin infection near the armpit, or a general illness like a cold, the flu, or mono. The swelling typically shows up on the same side as the infection. If you recently had a sore throat or upper respiratory infection, a swollen node in your armpit is your immune system doing its job. These nodes usually shrink back to normal within two to four weeks once the infection clears.
Vaccine-Related Swelling
If you’ve had a vaccination in the past few months, particularly an mRNA vaccine like a COVID-19 shot, the lump could be a lymph node reacting to the vaccine. This swelling appears on the same side as the injection arm. In a study of 111 patients who developed armpit swelling after COVID-19 vaccination, the average time to full resolution was about 127 days from the first dose. So if you’re a few weeks or even a couple months post-vaccination and still feeling a lump, that’s within the expected window.
Cysts and Infected Hair Follicles
Shaving, antiperspirant use, and friction from tight clothing can all irritate the delicate skin in your armpit. This irritation sometimes causes a hair follicle to become blocked or infected, producing a red, painful bump that feels like a marble under the skin. These are especially common in teens who are new to shaving. Allergic reactions to deodorants or creams can cause similar localized swelling.
A cyst in the armpit tends to feel firm, may be tender or swollen around the edges, and can become painful if it ruptures or gets infected. By contrast, a lipoma (a harmless fatty growth) feels soft and doughy, moves easily when you press on it, and is usually painless. Lipomas are typically smaller than two inches across and grow very slowly, if at all.
Hidradenitis Suppurativa
If you get painful lumps in your armpit repeatedly, especially ones that persist for weeks or months, this may be a condition called hidradenitis suppurativa. It starts when hair follicles become deeply blocked, leading to painful pea-sized lumps under the skin that heal slowly and tend to come back. Over time, some bumps break open and drain pus, sometimes with a noticeable odor. The condition favors areas where skin rubs together: armpits, groin, buttocks, and under the breasts.
Hidradenitis suppurativa is not just a boil, though it can look like one at first. The key difference is the pattern of recurrence and the potential for tunnels to form under the skin, leading to scarring. Hormonal changes during puberty may play a role, and it’s thought to involve an overactive immune response to clogged follicles. If this sounds familiar, it’s worth getting a diagnosis, because early treatment can prevent the condition from progressing.
Breast Tissue in the Armpit
Something many people don’t realize is that breast tissue extends into the armpit area. This extension, sometimes called the tail of Spence, connects to the central duct system of the breast. It can become more noticeable during pregnancy, breastfeeding, or at certain points in the menstrual cycle when hormonal shifts cause breast tissue to swell. If the lump seems to come and go with your period, or appeared during pregnancy or lactation, this is a likely explanation. The tissue responds to estrogen and progesterone just like the rest of the breast, so fluctuations are normal.
When a Lump Needs Medical Attention
Most armpit lumps are benign, but there are specific warning signs to watch for. A lump that is hard, doesn’t move when you press on it, and continues to grow over time has characteristics associated with cancer, including lymphoma and breast cancer. Painlessness isn’t necessarily reassuring here, since cancerous lumps are sometimes painful and sometimes not.
You should have a lump evaluated if it:
- Keeps getting bigger rather than shrinking over time
- Has been present for more than two to four weeks without an obvious cause like an infection or vaccine
- Feels hard and fixed in place rather than soft or mobile
- Appears alongside other symptoms like unexplained weight loss, night sweats, or changes in your breast
If imaging is needed, the typical workup involves an ultrasound of the armpit, sometimes paired with a diagnostic mammogram. Ultrasound is particularly good at revealing small abnormalities in lymph nodes. If anything looks suspicious on imaging, a needle biopsy of the node can determine whether cancer cells are present. This process sounds daunting, but for most people the lump turns out to be a reactive node or a benign growth, and the evaluation provides peace of mind.