Swollen lymph nodes in the armpit are most often caused by infections or injuries in the arm, hand, or chest area. Less commonly, they signal something more serious like cancer. A healthy armpit lymph node measures less than 10 millimeters across, so anything you can feel with your fingers is likely at least mildly enlarged. Understanding the range of causes can help you figure out what you’re dealing with and whether it needs attention.
What Armpit Lymph Nodes Actually Do
Your armpits contain clusters of lymph nodes that act as filters for the lymphatic system, a network of vessels that carries immune cells and fluid throughout your body. The axillary nodes specifically handle drainage from your arms, hands, chest wall, and breasts. When these nodes detect bacteria, viruses, or abnormal cells in the fluid passing through them, they ramp up immune activity and swell as a result. That swelling is your immune system working, not necessarily a sign something has gone wrong.
Infections Are the Most Common Cause
The vast majority of swollen armpit lymph nodes trace back to some kind of infection or injury in the areas they drain. A cut on your hand that gets infected, cellulitis on your arm, or even a bad hangnail can trigger swelling in the nearest armpit. The nodes are simply catching and fighting off whatever pathogen entered through the skin.
Certain specific infections are well known for targeting armpit nodes. Cat scratch disease, caused by bacteria transmitted through a cat’s scratch or bite on the hand or arm, frequently causes dramatic swelling in the armpit on the same side. Sporotrichosis, a fungal infection people pick up from handling soil, moss, or rose thorns, follows the same pattern. Mastitis, a breast tissue infection common during breastfeeding, can also cause armpit node swelling since those nodes filter fluid from the breast.
Systemic infections, meaning those affecting the whole body, can cause lymph nodes to swell in multiple areas at once, including the armpits. Mononucleosis, HIV, and certain other viral illnesses fall into this category. If you notice swelling in your armpits along with your neck and groin, a body-wide infection is more likely than a localized one.
Vaccines Can Cause Temporary Swelling
Vaccination is a common and completely harmless cause of armpit lymph node swelling that catches many people off guard. COVID-19 vaccines brought this into the spotlight, but flu shots and other injections given in the upper arm can do the same thing. The swelling typically appears on the same side as the injection because the immune response is strongest in the nodes closest to where the vaccine was delivered.
Radiologists now consider one-sided armpit swelling that appears within six weeks of a vaccination to be benign, with no imaging needed. If swelling persists beyond six weeks after your final dose, an ultrasound may be appropriate. Some imaging centers even reschedule routine mammograms to four to six weeks after vaccination to avoid false alarms on the scan.
Breast Cancer and Lymphoma
Cancer is the cause people worry about most, and while it’s far less common than infection, it’s worth understanding what to look for. Breast cancer can spread to armpit lymph nodes because those nodes directly filter fluid from breast tissue. When cancer cells take hold in a lymph node, they tend to change its internal structure: the fatty center of the node disappears, and the outer layer thickens unevenly. On the outside, cancerous nodes often feel hard, fixed in place (rather than movable under the skin), and painless.
Lymphoma, a cancer that starts in the lymphatic system itself, can also appear as swollen armpit nodes. These nodes grow over time rather than shrinking on their own, and they’re typically painless. The risk of cancer being the underlying cause increases with age, particularly in people with a personal or family history of breast cancer or lymphoma.
Painless swelling that doesn’t resolve is the pattern that raises the most concern. Infectious swelling tends to be tender and appears alongside other signs of illness, while malignant swelling is more likely to be firm, painless, and persistent.
Skin Conditions That Mimic Swollen Nodes
Not every lump in your armpit is actually a lymph node. Hidradenitis suppurativa is a chronic skin condition that creates small, painful lumps under the skin in areas where skin rubs together, and the armpit is one of the most common locations. These lumps start as pea-sized, painful bumps that persist for weeks or months. Over time, some break open and drain pus with an odor. The condition is not a boil, though it’s frequently misdiagnosed as one.
Hidradenitis suppurativa lumps sit in the skin itself rather than deeper in the tissue where lymph nodes live. They often appear in pairs, sometimes alongside blackheads in small pitted areas of skin. The condition can actually interfere with lymph drainage over time, causing additional swelling in the arms. Infected hair follicles, sebaceous cysts, and lipomas (soft fatty lumps) can also be mistaken for swollen lymph nodes in the armpit.
When Swelling Warrants Further Evaluation
Any palpable armpit lymph node is considered a red flag finding in clinical guidelines, regardless of size. That said, context matters enormously. If you recently had a vaccine in that arm, cut your hand, or are fighting off a cold, the swelling has an obvious explanation and will likely resolve on its own.
For newly swollen nodes without an obvious cause and no other worrying symptoms, observation for up to four weeks is considered safe. Most infection-related swelling will shrink within that window. If the node hasn’t returned to normal after three to four weeks, or if no clear cause has been identified, a biopsy or further workup is typically the next step.
Ultrasound is the recommended first-line imaging test for a new palpable armpit lump, whether it appears on one side or both. It can distinguish between a reactive lymph node (one that’s just responding to an infection), a node with suspicious internal changes, and lumps that aren’t lymph nodes at all. Your age, risk factors, and clinical picture all influence what comes after the ultrasound.
Signs That Point Toward Something Serious
Certain features should prompt you to seek evaluation sooner rather than waiting a full four weeks. A node that feels rock-hard or seems attached to the tissue around it rather than sliding freely under the skin is more concerning than a soft, movable one. Painless swelling that grows steadily over weeks deserves attention, especially if you also have unexplained weight loss, drenching night sweats, or persistent fevers. Swelling above the collarbone on either side is always treated as higher urgency than armpit swelling alone.
On the reassuring side, nodes that are tender, soft, and appeared suddenly alongside a visible infection or illness are behaving exactly the way healthy lymph nodes should. Tenderness, counterintuitively, is often a good sign. It means the node is actively fighting something rather than being quietly infiltrated by abnormal cells.