Swollen armpits are usually caused by enlarged lymph nodes reacting to a nearby infection or injury. Your armpits contain 20 to 40 lymph nodes, and they act as filters for your immune system, trapping bacteria and other threats from your arms, chest, and upper back. When those nodes detect something worth fighting, they swell with extra immune cells. Most of the time this is temporary and harmless, but certain features of the swelling can signal something that needs medical attention.
Infections and Injuries Are the Most Common Cause
A cut, scrape, or skin infection on your hand, arm, or chest is the single most frequent reason for armpit swelling. Even a minor wound you barely noticed can trigger a noticeable reaction in the nearest lymph nodes. Razor nicks from shaving, an ingrown hair that gets infected, or a bug bite on your forearm can all do it. The swelling typically appears within a day or two of the infection taking hold and shrinks once the infection clears.
Cat scratch disease is a well-known trigger. After a scratch or bite from a cat carrying the Bartonella henselae bacterium, a small red-brown bump appears at the wound site within 3 to 10 days. That bump fades over one to three weeks, but as it does, the lymph nodes draining that area swell up. More than 65% of cat scratch disease cases involve nodes in the armpit or neck. The swelling can persist for weeks and sometimes the node becomes quite tender, but the infection is generally self-limiting.
Skin Conditions That Mimic Swollen Nodes
Not every lump in your armpit is a lymph node. Two common skin conditions can create painful bumps in the same area, and they feel quite different from node swelling once you know what to look for.
Hidradenitis suppurativa starts as a single painful lump under the skin that can persist for weeks or months. It tends to recur in areas where skin rubs together or where sweat glands are concentrated: armpits, groin, buttocks, and under the breasts. Over time, bumps may fill with pus, break open and drain with an odor, and eventually form tunnels under the skin. The hallmark is a pattern of flare-ups and healing that leaves ropelike scars or pitted skin. Blackheads appearing in small, pitted pairs nearby are another telltale sign. This is a chronic inflammatory condition, not an infection, though individual bumps can become infected.
A simple cyst, by contrast, is usually a single smooth, round lump that moves slightly under the skin when you press on it. Cysts grow slowly, rarely drain on their own, and don’t form the tunneling or scarring pattern of hidradenitis. They can become tender if they get inflamed but typically aren’t painful otherwise.
Vaccine-Related Swelling
If you recently received a vaccination in your upper arm, the armpit on that same side can swell noticeably. This became widely recognized during the COVID-19 vaccine rollout, but it can happen after a flu shot or other injections too. The swelling is simply your immune system responding to the vaccine exactly as intended.
What surprises most people is how long it can last. Imaging guidelines from the Society of Breast Imaging were revised in 2022 to recommend waiting at least 12 weeks before pursuing follow-up imaging for vaccine-related armpit swelling, because research showed many nodes took that long or longer to return to normal size. If you notice armpit swelling within a few days of a vaccination on that side, the timing alone makes the cause fairly clear.
Autoimmune and Inflammatory Causes
Conditions like rheumatoid arthritis and lupus can cause lymph nodes to enlarge as part of widespread immune activation. Sarcoidosis and tuberculosis are less common but also known triggers. In these cases, the armpit swelling is rarely the only symptom. You’d typically notice joint pain, fatigue, rashes, or other systemic signs alongside it. Silicone breast implants can also cause armpit lymph node swelling through an inflammatory reaction to silicone particles if the implant leaks, even without visible changes to the breast itself.
When Swelling May Signal Cancer
This is the concern most people searching this topic carry in the back of their mind, so it’s worth being direct: the vast majority of swollen armpit nodes are not cancer. But when no infection, injury, or other clear trigger explains the swelling, the possibility of a malignant cause rises significantly.
Several cancers can cause armpit lymph nodes to enlarge. Lymphoma, both Hodgkin and non-Hodgkin types, often first appears as a painless swollen node. Breast cancer is another major consideration, as cancer cells from the breast commonly spread to axillary nodes first. Lung, thyroid, stomach, colorectal, pancreatic, ovarian, kidney, and skin cancers (particularly melanoma) can also metastasize to the armpit.
The texture and behavior of the node matter. Nodes that feel soft are generally insignificant. A rubbery feel is classically associated with lymphoma. Hard, fixed nodes that don’t move when you press them raise more concern for metastatic cancer. Painless swelling is actually more worrisome than tender swelling, since pain usually indicates your immune system is actively fighting an infection.
The systemic symptoms that accompany lymphoma are distinctive: drenching night sweats (not just feeling warm), unexplained weight loss, persistent fatigue, fevers without an obvious infection, itchy skin, and sometimes chest or bone pain. If your armpit swelling comes with several of these, that combination warrants prompt evaluation.
What the Size and Duration Tell You
A reactive node from a minor infection might swell to the size of a pea or small grape and shrink back within one to two weeks as the infection resolves. Nodes that keep growing over several weeks, reach a size you can easily see or feel without pressing, or don’t shrink after an infection clears deserve a closer look.
Doctors evaluating a swollen armpit node consider its size, how long it’s been present, whether it’s tender, how it feels (soft, rubbery, or hard), whether it moves freely or seems anchored to surrounding tissue, and whether there’s an obvious cause like a recent cut or vaccination. Imaging, typically ultrasound, is the usual next step when the cause isn’t clear. If imaging raises questions, a biopsy (removing a small sample of tissue from the node) provides a definitive answer.
What You Can Do Right Now
Check your hands, arms, and chest for any cuts, rashes, bites, or signs of skin infection. Look at whether the swelling is on one side or both, and whether it appeared suddenly or has been gradually growing. Think about recent vaccinations, new deodorants or razors, and any recent contact with cats.
A warm compress can ease discomfort from an inflamed node. If you can trace the swelling to an obvious cause like a healing cut or recent vaccine, giving it two to four weeks to resolve on its own is reasonable. Swelling that persists beyond a few weeks without explanation, continues to grow, feels hard or immovable, or comes alongside night sweats, unexplained weight loss, or persistent fatigue is worth getting evaluated sooner rather than later.