Swollen lymph nodes in the armpit are most often caused by infections in the arm, hand, or chest area, but they can also result from vaccinations, skin conditions, autoimmune disorders, and sometimes cancer. Your armpits contain clusters of lymph nodes that filter fluid draining from your upper body, so anything that triggers an immune response in that region can cause them to swell. Nodes under 10 mm in their shortest dimension are generally considered normal, while those over 10 mm warrant closer attention.
Bacterial and Viral Infections
The most common reason for swollen armpit nodes is a nearby infection. Lymph nodes along the lateral wall of the axilla receive fluid draining from the entire arm, so a cut on your hand, an infected hangnail, or even a bug bite that gets inflamed can cause noticeable swelling under your arm. Staphylococcus and Streptococcus bacteria are the most frequent culprits, causing skin infections like cellulitis or abscesses that trigger a strong response in the nearest nodes.
Viral illnesses can also enlarge armpit nodes, particularly infections like mononucleosis that cause widespread lymph node swelling throughout the body. In these cases you’ll typically notice swollen nodes in other areas too, like your neck or groin, along with fatigue and fever. The swelling usually resolves on its own as the infection clears, though it can take several weeks for nodes to return to their normal size.
Cat Scratch Disease
A less obvious but well-documented cause is cat scratch disease, an infection caused by Bartonella henselae bacteria transmitted through a cat scratch or bite on the hand or arm. The armpit nodes are among the most commonly affected because they sit along the drainage pathway from the upper limbs. Swelling can be significant, with nodes sometimes growing to over 30 mm, and may be accompanied by fever and pain that develops over days to weeks after the initial scratch. The swelling can look alarming enough on imaging to mimic lymphoma, which sometimes leads to a biopsy before the true cause is identified.
Vaccines
Vaccination is one of the more common and least concerning causes of armpit swelling, particularly after COVID-19 shots. Because most vaccines are injected into the upper arm, the nearby axillary nodes ramp up immune activity and can become noticeably enlarged. A prospective study tracking this effect found that complete resolution occurred at a median of six weeks, but in about half of participants, some degree of swelling was still visible on ultrasound at 12 weeks. A few people saw it linger up to 16 weeks before fully resolving. Flu shots and other arm-injected vaccines can produce the same effect, though typically less pronounced than what was seen with COVID-19 vaccines.
This kind of swelling is a normal sign that your immune system is responding to the vaccine. If you’re scheduled for a mammogram or breast imaging, it’s worth mentioning a recent vaccination so the radiologist doesn’t mistake the enlarged nodes for something more concerning.
Skin Conditions and Local Irritation
Hidradenitis suppurativa is a chronic skin condition that causes painful lumps under the skin in areas where skin rubs together, with the armpits being one of the most common sites. It typically starts with a single painful bump that persists for weeks or months, then progresses to recurring lumps. Because the armpits contain so many lymph nodes, the chronic inflammation and scarring from hidradenitis can interfere with lymph drainage and cause persistent swelling in the arms.
Less dramatically, razor bumps, ingrown hairs, or irritation from deodorant products can introduce bacteria into small breaks in the skin. This localized infection may be minor enough that you barely notice it on the skin’s surface, but the nearby lymph nodes still swell in response. These cases usually resolve once the skin heals.
Autoimmune Conditions
Diseases where the immune system attacks the body’s own tissues can cause lymph nodes to enlarge throughout the body, including the armpits. Lupus, rheumatoid arthritis, and dermatomyositis are all associated with generalized lymph node swelling. Sarcoidosis, a condition that causes clusters of inflammatory cells to form in various organs, is another recognized cause of axillary node enlargement. In these cases, the swollen nodes are one piece of a larger pattern that includes joint pain, skin changes, fatigue, or other systemic symptoms.
Breast Cancer and Lymphoma
Cancer is the cause most people worry about, and while it’s far less common than infection, it’s the reason persistent or unusual node changes deserve medical attention. Breast cancer can spread to the axillary lymph nodes, and the risk of this increases with tumor size. Tumors larger than 40 mm are roughly three times more likely to involve the armpit nodes than smaller ones. This is why checking the axillary nodes is a standard part of breast cancer staging.
Lymphoma, a cancer that originates in the lymphatic system itself, can also present as enlarged armpit nodes. The physical characteristics of the node offer some clues. Nodes caused by infection tend to be soft and tender. Lymphoma nodes often feel firm and rubbery. Metastatic cancer nodes are more likely to feel hard, almost stonelike, and may be fixed in place rather than moving freely under the skin. Clusters of nodes that seem fused together can also suggest malignancy, though this isn’t definitive on its own. Pain alone is not a reliable way to distinguish between benign and cancerous nodes, since both can be painful or painless.
How to Tell What’s Concerning
Most swollen armpit nodes are reactive, meaning they’re responding to a nearby infection or immune trigger and will shrink back to normal within a few weeks. A few features should prompt you to get checked sooner rather than later:
- Duration: Nodes that persist beyond two weeks without an obvious cause like a recent vaccine or healing wound.
- Texture: Nodes that feel very hard or seem stuck to surrounding tissue rather than sliding under your fingers.
- Size: Nodes that keep growing or exceed 15 mm, particularly without pain or other signs of infection.
- Other symptoms: Unexplained weight loss, night sweats, or persistent fatigue alongside the swelling.
A single soft, tender node that shows up after a cut on your hand or a recent flu shot is almost certainly doing exactly what it’s supposed to do. Multiple hard, painless nodes that appear without explanation tell a different story. The context surrounding the swelling matters as much as the swelling itself.