Swollen lymph nodes in the armpit are most often caused by an infection in the arm, hand, or breast area. Lymph nodes act as filters that trap germs and abnormal cells, and they enlarge when your immune system is actively fighting something off. In the vast majority of cases, the swelling is temporary and resolves on its own, but certain features of the swelling can signal something that needs medical attention.
Infections Are the Most Common Cause
Your armpit contains a dense cluster of lymph nodes (called axillary nodes) that drain fluid from your arms, chest wall, and breasts. Any infection in these areas can trigger swelling. A cut on your hand, an infected hangnail, cellulitis on your arm, or even a dental or throat infection can cause one or more armpit nodes to enlarge as they filter out bacteria.
Cat scratch disease is a well-known trigger: bacteria from a cat scratch or bite on the hand or arm travel to the nearest lymph nodes, often in the armpit, causing them to swell significantly. Bacterial skin infections caused by staph or strep are another frequent culprit. Viral infections like mononucleosis (Epstein-Barr virus), HIV, and CMV can cause more widespread swelling that includes the armpits along with the neck and groin. Tuberculosis and syphilis are less common but documented infectious causes.
Infected nodes from a bacterial source tend to be tender, warm, and soft. They often come on quickly and may be accompanied by redness or streaking on the skin nearby.
Shaving, Antiperspirants, and Skin Irritation
Cysts or abscesses under the skin of the armpit can produce large, painful lumps that feel similar to swollen lymph nodes. These are commonly caused by shaving or the use of antiperspirants, which can clog pores or introduce bacteria through tiny nicks in the skin. The result is a localized infection or an inflamed hair follicle that may swell enough to be mistaken for a lymph node problem.
If you recently changed your shaving routine or switched products, this is worth considering before assuming the worst. These lumps are typically right at the skin surface and very tender to the touch.
Vaccines Can Cause Weeks of Swelling
Vaccination is one of the most common and most misunderstood causes of armpit lymph node swelling. COVID-19 vaccines in particular are well documented to cause enlargement of the axillary nodes on the same side as the injection. This is a normal immune response, not a sign of a problem, but it can last much longer than most people expect.
Research published in the American Journal of Roentgenology found that after a COVID-19 booster dose, axillary lymph node swelling took an average of 102 days to fully resolve on ultrasound. After the initial two-dose series, resolution took even longer, averaging 129 days. In some cases, swelling persisted on imaging for up to 43 weeks. Radiologists now recommend waiting at least 12 weeks before investigating vaccine-related swelling further, and suggest not delaying routine mammograms if vaccine-related swelling is the suspected cause.
Flu shots and other vaccines given in the upper arm can cause similar (though typically shorter-lived) swelling.
Breast Cancer and Other Malignancies
The axillary lymph nodes are closely connected to breast tissue, which is why swollen armpit nodes are part of the standard evaluation for breast cancer. Cancer cells that break free from a tumor in the breast may travel through these nodes on the way to other parts of the body. A painless, hard lump in the armpit that doesn’t go away can be an early sign of breast cancer, even before a mass is felt in the breast itself.
Lymphoma, both Hodgkin and non-Hodgkin types, can also present as swollen armpit nodes. These nodes tend to feel rubbery and firm rather than soft, and they’re usually painless. Melanoma or other skin cancers on the arm, back, or chest can spread to the axillary nodes as well.
The features that raise concern for cancer are distinct from those of infection. A node that feels hard, irregular, or fixed in place (it doesn’t move when you push it) is more suspicious than one that’s soft and mobile. Other red flags include swelling that lasts longer than four to six weeks, nodes that haven’t returned to normal size after eight to twelve weeks, unexplained weight loss, night sweats, and persistent fevers.
Autoimmune and Inflammatory Conditions
Systemic lupus erythematosus (lupus) frequently causes lymph node swelling, and the armpits are one of the most commonly affected areas alongside the neck and groin. In lupus, swollen nodes tend to be soft, not tender, and often appear during disease flares alongside other symptoms like joint pain, fatigue, and skin rashes. Sarcoidosis, another inflammatory condition, can also cause generalized lymph node enlargement that includes the axillary region.
Rheumatoid arthritis and other connective tissue diseases occasionally trigger reactive lymph node swelling, particularly during periods of high inflammation. If you have a known autoimmune condition and notice new swelling, it may reflect increased disease activity rather than a new problem.
How Doctors Evaluate Swollen Armpit Nodes
During a physical exam, doctors assess several characteristics of the node. Soft nodes generally point to something benign. Rubbery, firm nodes are classically associated with lymphoma. Hard, irregular nodes raise concern for cancer or certain granulomatous infections like tuberculosis. Tenderness typically points toward infection, while painless enlargement is more associated with malignancy.
On ultrasound, radiologists look at the node’s internal structure. A normal axillary lymph node has a bright fatty center (called a hilum) and a thin outer layer (cortex) measuring less than 3 mm. When the cortex thickens beyond 3 mm, or when the fatty hilum disappears entirely, the node is considered suspicious for metastatic disease and may warrant a needle biopsy.
If cancer seems unlikely based on the exam, doctors will typically recommend watching the node for about four weeks to see if it shrinks on its own. Nodes that persist beyond four to six weeks, continue to grow, or develop additional concerning features are usually biopsied. This is done with a fine-needle aspiration or a core needle biopsy, both of which are relatively quick outpatient procedures.
What the Swelling Feels Like Matters
The characteristics of your swollen node tell you a lot. A small, tender, movable lump that showed up alongside a cold, a skin cut, or a recent vaccine is overwhelmingly likely to be harmless. A painless, hard, immobile lump that has been growing for weeks carries a higher index of concern, especially if you’re over 40 or have systemic symptoms like unexplained weight loss, drenching night sweats, or persistent fevers.
Swelling in both armpits (or in the armpits plus the neck or groin) suggests a systemic process: a widespread infection, an autoimmune condition, or sometimes lymphoma. Swelling limited to one armpit more commonly reflects something local, like an infection in that arm or a vaccine given on that side.