A swollen lymph node in your armpit is usually a sign that your body is fighting an infection in your arm, hand, or chest area. In most cases, the swelling is temporary and harmless. Less commonly, it can signal something more serious like lymphoma or breast cancer, especially when the node is hard, painless, and doesn’t go away after a few weeks.
Why the Armpit Is a Common Spot
Your armpits contain a dense cluster of lymph nodes that act as filters for a large portion of your upper body. These nodes collect and process fluid draining from your arms, hands, most of the breast tissue, the front and back of your chest wall, and the upper back including the shoulder blade area. That’s a lot of territory, which means a problem almost anywhere in your upper body can show up as a swollen node in your armpit.
Lymph nodes are small, bean-shaped structures packed with immune cells. When they detect bacteria, viruses, or abnormal cells in the fluid passing through, they ramp up their activity and swell. A normal armpit lymph node is typically too small and soft to feel. Once you can feel it, something has triggered an immune response.
Infections Are the Most Common Cause
The vast majority of swollen armpit nodes are caused by infections or minor injuries in the areas that drain into them. A cut on your hand, a skin infection on your arm, an ingrown hair, or even a razor nick in your armpit can be enough. The swelling means your immune system noticed the problem and is responding normally.
Some specific infections are particularly associated with armpit swelling. Cat scratch disease, caused by bacteria transmitted through a cat’s scratch or bite on the hand or arm, is a classic example. Sporotrichosis, a fungal infection that enters through skin breaks (common in gardeners), follows the same drainage pathway into the armpit nodes. Breast infections, including those related to breastfeeding, also frequently cause armpit swelling on the affected side.
Viral illnesses can cause more widespread lymph node swelling that includes the armpits. Mononucleosis, HIV, and certain other viral infections may cause nodes to enlarge in multiple areas of the body at once.
Vaccines Can Cause Temporary Swelling
If you recently received a vaccination in your upper arm, a swollen lymph node on that side is a normal immune response. COVID-19 vaccines, flu shots, and other injections can all trigger this. The swelling typically appears within days of the shot and resolves on its own over the following weeks. This is worth keeping in mind because it can cause unnecessary alarm, particularly if you happen to have a mammogram scheduled around the same time.
What a Concerning Node Feels Like
Not all swollen nodes carry the same level of concern. The physical characteristics of the node itself offer important clues about what’s going on.
A node that’s tender, soft, and movable under your fingers is more likely to be reacting to an infection. It hurts because it’s inflamed and working hard. This type of swelling often resolves within two weeks as the underlying infection clears.
A node that feels firm or hard, doesn’t move easily when you press on it (as if it’s stuck to surrounding tissue), and isn’t painful is more concerning. These characteristics are associated with a higher risk of malignancy. Nodes that keep growing over several weeks without an obvious cause also warrant closer attention.
Timing matters too. Swelling that lasts less than two weeks or more than a year without getting bigger has a very low likelihood of being cancerous. The window of concern falls in between: a node that persists for several weeks, keeps enlarging, and has no clear explanation.
When Swelling May Point to Cancer
Lymphoma, a cancer of the lymphatic system, commonly presents as painless swollen nodes in the neck, armpit, or groin. But the swelling alone isn’t enough for a diagnosis. Lymphoma typically comes with additional systemic symptoms: drenching night sweats (not just feeling warm), unexplained weight loss, persistent fatigue, fever without an obvious infection, itchy skin, or chest and bone pain.
Breast cancer can also spread to the armpit lymph nodes, since these nodes filter fluid from breast tissue. Occasionally, a swollen armpit node is the first noticeable sign of breast cancer before a lump is felt in the breast itself. This is one reason persistent, painless armpit swelling in adults deserves medical evaluation.
Less Common Causes
Some autoimmune and inflammatory conditions cause chronic lymph node swelling. Lupus, rheumatoid arthritis, and sarcoidosis can all produce enlarged nodes as part of a broader pattern of immune system overactivity. A rare condition called autoimmune lymphoproliferative syndrome causes persistent swelling in the neck, armpit, and groin that lasts for months or longer. In these cases, the swelling is typically present on both sides of the body and accompanied by other symptoms like joint pain, rashes, or fatigue.
Certain medications can also cause lymph node enlargement as a side effect. Some seizure medications and antibiotics are known triggers.
How to Check Your Armpit Nodes
You can feel for swollen armpit nodes at home using a simple technique recommended by the British Association of Dermatologists. Remove your top clothing so you can access both armpits freely. To check the left side, raise your left arm slightly, then place the fingers of your right hand high into the armpit. Lower your arm back down and gently press your fingers in a circular motion, feeling the central area of the armpit first.
Then move your fingers firmly along the front border of the armpit, along the back border, and down the inner side of your upper arm. Repeat the process on the other side. You’re feeling for any lumps, firm spots, or areas that feel different from one armpit to the other. Healthy nodes are usually too small and soft to detect, so anything distinctly palpable is worth noting.
What Happens During Medical Evaluation
If a swollen armpit node doesn’t resolve on its own within a few weeks, or if it has concerning characteristics, a doctor will typically start with a physical exam and a detailed history. They’ll ask about recent infections, injuries, vaccinations, and symptoms like night sweats or weight loss. They’ll feel the node and assess its size, texture, and mobility.
If the cause isn’t obvious, the next step is usually an ultrasound. Imaging can reveal internal details about the node’s structure. A node with a thin, even outer layer (the cortex) and a visible fatty center is more likely benign. A node with a thickened cortex measuring 3 mm or more, or one that’s lost its normal fatty center entirely, raises more suspicion and may lead to a biopsy. During a biopsy, a small sample of tissue is removed with a needle so it can be examined under a microscope for cancer cells or signs of specific infections.
Blood work may also be ordered to check for infections, immune system abnormalities, or markers that suggest a systemic process. The specific tests depend on the overall clinical picture. In many cases, the evaluation is reassuring and the node turns out to be a normal response to something minor that may have already resolved by the time you’re seen.