Swollen lymph nodes in the armpit are most often caused by infections, either in the arm or hand, or somewhere else in the body. Your armpits contain 20 to 40 lymph nodes that filter fluid from your arms, chest, and upper back, so they react to a wide range of triggers. Less commonly, the swelling points to an autoimmune condition, a reaction to a vaccine, or cancer.
Bacterial and Viral Infections
The most frequent culprits are everyday bacterial infections. A cut, scrape, or ingrown hair on your hand, arm, or chest can introduce bacteria (typically staph or strep) that travel through your lymphatic system and trigger inflammation in the nearest nodes. The nodes swell because they’re producing immune cells to fight the infection. You’ll usually notice tenderness, warmth, and sometimes redness over the swollen area, and the nodes tend to shrink once the underlying infection clears.
Systemic viral infections can also do it. Mono, flu, HIV, and other widespread viruses cause lymph nodes to enlarge in multiple areas at once, including the armpits. If you notice swollen nodes on both sides of your body or in your neck and groin at the same time, a virus is a likely explanation.
Cat Scratch Disease
If you’ve recently been scratched or bitten by a cat, a bacterium called Bartonella henselae may be the cause. Cat scratch disease typically starts with a small bump at the scratch site, followed days or weeks later by swelling in the nearest lymph nodes. The armpit is a common location when the scratch is on the hand or arm. Most cases are diagnosed based on symptoms and exposure history alone, and the swelling usually resolves on its own over several weeks, though nodes can stay enlarged for months in some people.
Vaccine Reactions
Vaccines injected into the upper arm can cause lymph node swelling on the same side, and this became widely recognized during COVID-19 vaccination. After an mRNA booster dose, swelling was detectable on ultrasound within a median of 6 days. It took an average of about 3 months to fully resolve after a booster, and even longer after the initial vaccination series, where swelling persisted for up to 43 weeks in some cases.
This reaction isn’t unique to COVID vaccines. Flu shots, shingles vaccines, and other upper-arm injections can trigger the same response. The swelling is simply your immune system responding to the vaccine and is not a sign of disease. If you’re scheduled for a mammogram or breast imaging, it’s worth mentioning a recent vaccination so that enlarged nodes aren’t mistaken for something more concerning.
Autoimmune and Inflammatory Conditions
Chronic swelling that comes and goes, especially alongside joint pain or fatigue, can signal an autoimmune condition. Rheumatoid arthritis is a notable example: roughly 75% of people with RA develop swollen lymph nodes at some point, and the armpit is one of the most commonly affected areas. Lupus and sarcoidosis can also cause lymph node enlargement. In these cases the swelling tends to be milder, less tender, and present in more than one region of the body.
Breast Cancer and Lymphoma
Cancer is a less common but more serious cause. Breast cancer can spread to the armpit lymph nodes, sometimes producing a painless lump that feels hard or fixed in place. In some cases, a swollen armpit node is the first noticeable sign of breast cancer, even before a lump is felt in the breast itself. Lymphoma, a cancer that starts in the lymph nodes, can also appear first in the armpit.
Cancerous nodes behave differently from infected ones. They tend to be painless, grow steadily over weeks, and don’t shrink with time or antibiotics. On imaging, a normal armpit node is oval, under 2 cm, and has a visible fatty center. Suspicious features include a thickened outer layer (greater than 3 mm), an irregular shape, or loss of that fatty center entirely. Nodes with focal, uneven thickening are the most suspicious for metastasis.
How to Tell It Apart From a Skin Condition
Not every lump in the armpit is a lymph node. Hidradenitis suppurativa, a condition where hair follicles become blocked, creates painful pea-sized lumps under the skin in areas that rub together, including the armpits. These bumps tend to recur in the same spots, may drain pus with an odor, and can eventually form tunnels under the skin connecting multiple lumps. Blackheads appearing in pairs nearby are another hallmark. Infected cysts and boils from shaving or antiperspirant irritation are also common armpit lumps that have nothing to do with lymph nodes.
A true swollen lymph node usually feels like a smooth, rubbery, movable ball deeper under the skin. A blocked hair follicle or cyst tends to sit closer to the surface and may develop a visible head or drain on its own.
When Swelling Deserves Attention
Most swollen armpit nodes resolve within two to four weeks once the triggering infection clears. Swelling that sticks around longer, keeps growing, or is accompanied by unexplained weight loss, night sweats, or fever warrants further evaluation. A node larger than 2 cm, one that feels hard or doesn’t move when you press on it, or one that appeared without an obvious infection or recent vaccination is worth getting checked. The usual next step is an ultrasound, which can assess the node’s shape and internal structure. If the imaging looks abnormal and there’s no clear explanation from your medical history, a biopsy provides a definitive answer.