Enlarged lymph nodes under the armpit are most often caused by infections, but they can also result from vaccines, skin irritation, autoimmune conditions, or cancer. A normal armpit lymph node measures less than 10 mm in diameter. When nodes swell beyond that size, it means your immune system is responding to something, whether that’s a minor cut on your hand or something more serious.
Your armpits contain five groups of lymph nodes that collectively filter fluid from your arms, upper back, chest wall, and the outer portions of your breasts. Because they serve as a drainage hub for so much of your upper body, these nodes can react to problems originating in many different areas.
Infections Are the Most Common Cause
The majority of swollen armpit lymph nodes are caused by bacterial, viral, or fungal infections. When germs enter your body through a wound on your hand, a skin infection on your arm, or even an infected hair follicle near your armpit, the nearby lymph nodes ramp up their immune response and swell as they work to trap and destroy the invaders.
Common infectious triggers include upper respiratory viruses, skin infections like cellulitis, infected cuts or scrapes on the arm or hand, and cat-scratch disease (a bacterial infection spread through a cat’s scratch or bite). Mononucleosis and HIV can also cause widespread lymph node swelling that includes the armpits. Infected nodes from bacterial causes tend to be tender, warm, and sometimes red on the overlying skin. Viral infections more commonly cause smaller, mildly tender nodes on both sides of the body rather than just one armpit.
Vaccines Can Cause Temporary Swelling
Vaccination is one of the most common and most overlooked causes of armpit lymph node swelling. Because most shots are given in the upper arm, the armpit nodes on that same side often react. This has been especially well documented with COVID-19 vaccines, but flu shots and other immunizations can do the same thing.
After a COVID-19 booster dose, swelling typically shows up within about a week, though some people don’t notice it for several weeks. The more surprising finding is how long it can take to fully resolve. In one study, the average time from vaccination to complete resolution on ultrasound was about 102 days (roughly three and a half months) after a booster and even longer, around 129 days, after a first dose. This is why doctors generally recommend waiting at least 12 weeks before pursuing further workup for armpit swelling that appears after vaccination. If you recently had a vaccine in that arm, the timing alone can explain the lump you’re feeling.
Skin Conditions and Local Irritation
Not every lump in your armpit is actually a lymph node. Hidradenitis suppurativa, a condition that causes small, painful bumps under the skin in areas where skin rubs together, frequently affects the armpits. These bumps are often mistaken for swollen lymph nodes or even pimples. Over time, hidradenitis suppurativa can create scar tissue that interferes with normal lymph drainage, leading to additional swelling in the arms.
Shaving nicks, ingrown hairs, and minor skin infections from cuts can also trigger nearby lymph node swelling. This kind of reactive swelling is usually mild, tender, and resolves within a couple of weeks once the skin heals. If you notice a swollen node shortly after shaving or getting a cut on your arm, that connection is worth considering before assuming something more serious.
Autoimmune Diseases
Systemic inflammatory conditions like lupus and rheumatoid arthritis can cause lymph node swelling throughout the body, including the armpits. In lupus, this is sometimes called lupus lymphadenitis, where the immune system’s chronic overactivity causes inflammation within the nodes themselves. The swelling in these cases tends to come and go alongside disease flares and is usually accompanied by other symptoms like joint pain, fatigue, or skin rashes. If you already have an autoimmune diagnosis, armpit node swelling during a flare is a recognized part of the disease rather than a separate problem.
Breast Cancer and Lymphoma
Cancer is the cause most people worry about, and while it’s far less common than infections, the armpit is a location where certain cancers do show up. Breast cancer frequently spreads to axillary lymph nodes because these nodes directly drain the breast tissue. Melanoma on the arm, chest, or upper back can also spread to armpit nodes. Lymphoma, a cancer that starts in the lymph nodes themselves, can appear as a painless, growing lump in one or both armpits.
The physical characteristics of the node offer clues. Nodes related to infection tend to be tender, soft, and easy to move around under the skin. Nodes associated with cancer are more likely to be hard, painless, and fixed in place. A rubbery texture that moves slightly is a classic description of lymphoma. On imaging, suspicious nodes lose their normal oval shape and become round, and the fatty center visible in healthy nodes gets replaced by abnormal tissue. Doctors look specifically for thickening of the node’s outer layer: focal or uneven thickening greater than 3 mm is concerning, and areas exceeding 6 mm are considered highly suspicious, often prompting a needle biopsy.
Warning Signs That Need Attention
A single tender, marble-sized node that appears after a cold or a cut and shrinks over two weeks is rarely anything to worry about. The pattern that warrants prompt evaluation looks different. Nodes that are hard, painless, and continue to grow over several weeks without an obvious infection are the ones that need investigation.
Certain whole-body symptoms alongside armpit swelling raise the concern for lymphoma or another serious cause. These include:
- Unexplained weight loss without changes in diet or exercise
- Drenching night sweats that soak through clothing or sheets
- Persistent fever without an identifiable infection
- Ongoing fatigue that doesn’t improve with rest
These are sometimes called “B symptoms” in the context of lymphoma, and their presence signals that the body is dealing with something systemic rather than a localized reaction.
How Doctors Evaluate Swollen Armpit Nodes
Evaluation typically starts with a physical exam and your recent history. Your doctor will want to know about recent infections, vaccinations, skin injuries, animal scratches, and travel. They’ll feel the node to assess its size, texture, tenderness, and mobility.
If the node doesn’t have an obvious explanation or doesn’t resolve in a few weeks, ultrasound is usually the next step. On ultrasound, a healthy lymph node looks oval with a visible fatty center and a thin outer layer. Warning signs include a rounded shape, loss of that fatty center, and thickening of the outer cortex. When imaging looks suspicious, doctors perform an ultrasound-guided needle biopsy, targeting the thickest or most abnormal-looking part of the node to get cells for analysis under a microscope.
For nodes that appear after a recent vaccination, the standard approach is watchful waiting for at least 12 weeks. Most vaccine-related swelling resolves on its own within that window, sparing you from unnecessary biopsies.