Swollen lymph nodes in the armpit are most commonly reactive, meaning your immune system is responding to an infection or irritation nearby. In the vast majority of cases, the cause is something routine like a skin infection, a recent illness, or even a vaccination. That said, the location, duration, and accompanying symptoms matter, so understanding what’s behind the swelling helps you know when it’s worth getting checked out.
What Armpit Lymph Nodes Actually Do
You have clusters of lymph nodes throughout your body, but the ones in your armpit (called axillary nodes) filter fluid draining from your arms, chest wall, and breast tissue. When something triggers an immune response in any of those areas, the nodes swell as they produce extra white blood cells to fight the threat. This is normal immune function, not a disease in itself.
The Most Common Causes
Bacterial skin infections are one of the top reasons for armpit lymph node swelling. A cut, ingrown hair, or infected razor nick on your arm or chest can trigger a response. Staph and strep bacteria are the usual culprits. If you can find an obvious wound, red streak, or area of warmth on the same side as the swollen node, that’s likely your answer.
Cat scratch disease is another surprisingly common cause of armpit swelling specifically. A scratch or bite from a cat (especially a kitten) on your hand or arm can lead to a bacterial infection that causes the nearest lymph nodes to enlarge, sometimes dramatically.
Viral infections often cause swelling in multiple lymph node groups at once. Epstein-Barr virus (the cause of mono) typically involves nodes in the neck, armpits, and groin on both sides. HIV can also cause generalized swelling, frequently starting in the neck, armpits, and back of the head. If you notice swollen nodes in several locations, a systemic infection is more likely than a local problem.
Vaccines Can Cause It Too
If you recently received a vaccine in your upper arm, that alone can explain swollen nodes on the same side. mRNA COVID vaccines brought widespread attention to this, but flu shots and other arm-injected vaccines can do it as well. The swelling is your immune system reacting to the vaccine exactly as intended.
Most vaccine-related lymph node swelling improves within 30 to 45 days. In some cases, though, it can take longer. One documented case took roughly six months for the node to return to near-normal size. If swelling persists well beyond a month after vaccination, it’s reasonable to have it evaluated, but knowing you were recently vaccinated gives your doctor important context.
Autoimmune and Inflammatory Conditions
Lupus frequently causes enlarged lymph nodes in the neck, armpits, and groin. This swelling tends to come and go with disease flares and is usually present on both sides. Sarcoidosis, a condition that causes clusters of inflammatory cells to form in various organs, can also present with armpit lymph node enlargement. In both cases, the swelling is typically one piece of a larger picture that includes fatigue, joint pain, or other systemic symptoms.
When Swelling Could Signal Something Serious
Most swollen armpit nodes are harmless, but certain features raise concern. The combination of unexplained fevers, drenching night sweats, and unintentional weight loss (more than 10% of your body weight) is a recognized pattern associated with lymphoma. Being over 40, male, or having nodes that keep growing rather than shrinking also increases the statistical likelihood of a more serious cause.
Unilateral armpit swelling (one side only) with no obvious infection or recent vaccination raises more suspicion than bilateral swelling, because it can be associated with breast cancer on the same side. That said, the physical feel of a node is not a reliable way to judge what’s happening inside it. Research shows that size alone and whether a node is painful or painless have no predictive value for malignancy. What matters more is what the node looks like internally on imaging.
What Happens During Evaluation
If your doctor wants a closer look, the first step is usually an ultrasound. Radiologists evaluate the node’s shape, the thickness of its outer layer (the cortex), and whether the fatty center (the hilum) is intact. A normal lymph node appears oval with a thin, even cortex and a bright fatty center.
Nodes classified as low suspicion may show cortical thickening of 3 millimeters or more, but this finding alone is nonspecific since both infections and cancer can cause it. The most telling sign of malignancy is when the fatty hilum is completely replaced or absent, giving the node a uniformly dark, round appearance on ultrasound. Loss of the hilum has a positive predictive value for malignancy between 58% and 97%, making it the most specific imaging feature. When both sides show symmetrically abnormal nodes, that pattern points more toward a blood cancer like lymphoma or leukemia than a solid tumor.
If imaging raises concern, a biopsy (usually a needle biopsy guided by ultrasound) provides a definitive answer.
What to Do Right Now
Start by looking for an obvious explanation. Check the arm, hand, and chest on the same side for cuts, insect bites, rashes, or signs of infection. Think about whether you’ve been sick recently or received a vaccine in that arm within the past few weeks. A single, tender node that appeared after a clear trigger is almost always benign and will resolve on its own.
If you can’t identify a cause, give it about two weeks. Most reactive swelling starts to improve in that window. MD Anderson Cancer Center recommends seeing your doctor if the swelling has no clear explanation, continues to grow, or hasn’t improved after two weeks. You don’t need to panic during that waiting period, but you shouldn’t ignore nodes that are steadily getting larger or that come with fevers, night sweats, or unexplained weight loss.