A ball or lump in your armpit is usually a swollen lymph node, and the most common cause is an ordinary infection like a cold, a skin irritation, or a recent vaccine. Your armpit contains 20 to 40 lymph nodes, small bean-shaped glands that filter germs and can puff up when your immune system is working hard. Less often, the lump is something else entirely: a cyst, a fatty deposit, an ingrown hair, or a blocked sweat gland. Rarely, it signals something more serious.
Swollen Lymph Nodes From Infection
Lymph nodes in the armpit sit close to the surface, so even a minor infection in your arm, hand, chest, or breast can make one swell large enough to feel like a marble or grape under the skin. Common triggers include colds, strep throat, ear infections, mononucleosis, and skin infections like cellulitis. An infected cut or scratch on your hand or forearm is enough. The node typically feels soft, moves when you push it, and may be tender.
Less common infections can also be responsible. Cat scratch fever, caused by bacteria from a cat scratch or bite, frequently causes armpit swelling on the same side as the wound. Sexually transmitted infections like syphilis, as well as tuberculosis and toxoplasmosis, can produce enlarged nodes too, though these are far less likely for most people.
In most cases, the node shrinks on its own once the underlying infection clears. If a swollen node hasn’t gone down after two weeks, that’s a reasonable point to get it checked.
Shaving, Ingrown Hairs, and Folliculitis
If the bump is right at the skin surface and appeared after shaving, it’s likely folliculitis or an ingrown hair. Folliculitis shows up as clusters of small, pimple-like bumps around hair follicles. They can itch, burn, or feel tender, and sometimes fill with pus that breaks open and crusts over. Ingrown hairs look similar but happen when a shaved hair curls back into the skin and triggers inflammation rather than true infection. People with curly hair are especially prone.
Mild cases heal on their own within a few days with basic care: warm compresses, gentle cleansing, and avoiding shaving the area until it calms down. If the bumps spread or persist beyond a week or two, a doctor can determine whether you need a topical treatment.
Cysts, Lipomas, and Other Lumps
Not every armpit ball is a lymph node. A lipoma is a slow-growing pocket of fat just under the skin. It feels soft and doughlike, usually measures less than two inches across, and slides around freely when you press on it. Lipomas are almost always painless unless they sit against a nerve or near a joint. They can show up anywhere on the body, including the armpit, and are benign.
Sebaceous cysts, which develop when a hair follicle’s oil gland gets blocked, tend to feel firmer than lipomas and may have a small dark dot at the center. They’re generally harmless but can become infected, turning red, painful, and swollen.
Hidradenitis Suppurativa
If the lump keeps coming back, or you develop multiple painful bumps in areas where skin rubs together (armpits, groin, buttocks, under the breasts), you may be dealing with hidradenitis suppurativa. This chronic condition starts with a single painful lump under the skin that can persist for weeks or months. Over time, some bumps break open and drain foul-smelling pus. Paired blackheads in small pitted areas of skin are another hallmark. Without treatment, the lumps can create tunnels under the skin and lead to scarring. It’s worth seeing a dermatologist early, because treatment is more effective before the condition progresses.
Vaccine Reactions
A lump in the armpit on the same side where you recently got a shot is a well-documented reaction, particularly after COVID-19 vaccines. The swelling typically appears within a few days of vaccination, though it can show up weeks later. In a study published in the American Journal of Roentgenology, post-booster armpit swelling took an average of about 102 days from the vaccination date to fully resolve, with most cases clearing within a few months. This type of swelling is your immune system responding to the vaccine exactly as designed. If you’re scheduled for a mammogram or breast ultrasound, mention the recent vaccination so the radiologist can factor it in.
Deodorant and Contact Dermatitis
Sometimes the “ball” is really a patch of inflamed, bumpy skin from an allergic reaction. Contact dermatitis from deodorants or antiperspirants shows up as an itchy, red, flaking, or bumpy rash right where the product was applied. Switching to a fragrance-free or aluminum-free formula often resolves it. If the irritation doesn’t improve within a couple of weeks of stopping the product, a dermatologist can help identify the specific ingredient causing the reaction.
When an Armpit Lump Could Be Serious
The vast majority of armpit lumps are harmless, but certain features raise concern for breast cancer or lymphoma. A lump that feels hard, doesn’t move when you push it, and is painless is more worrisome than one that’s soft, mobile, and tender. Lumps related to cancer tend to grow steadily over time rather than appearing suddenly with an infection and then shrinking.
Unilateral swelling (one armpit only, with no clear cause like a nearby infection or recent vaccine) raises more suspicion than swelling on both sides, which is more typical of systemic infections or, less commonly, lymphoma. Other red flags include unexplained weight loss, drenching night sweats, persistent fevers, and a lump that keeps getting bigger over weeks.
On imaging, normal armpit lymph nodes are oval, less than 10 millimeters across, with a thin outer layer under 3 millimeters. Nodes that become round, lose their normal internal fat structure, or develop a thickened outer layer are flagged for further evaluation. If your doctor orders an ultrasound and something looks suspicious, the next step is usually a needle biopsy to sample the tissue.
What to Expect at the Doctor
For a new armpit lump, the first step is typically a physical exam. Your doctor will note the size, texture, mobility, and tenderness of the lump and ask about recent infections, vaccines, shaving habits, and any other symptoms. For women, an ultrasound of the armpit is the standard initial imaging test. If that ultrasound reveals a suspicious-looking node, a needle biopsy can be done during the same visit or shortly after. The procedure takes minutes, and results usually come back within a few days.
A lump that hasn’t gone away after two weeks, feels hard and painful, keeps growing, or comes with a fever is worth having evaluated. One that’s clearly linked to a recent cold, vaccine, or shaving irritation and is already shrinking can safely be monitored at home.