Swollen lymph nodes are almost always a sign that your immune system is responding to something, most commonly an infection. These small, bean-shaped glands filter fluid and trap bacteria, viruses, and other threats. When they detect a problem, immune cells multiply rapidly inside the node, causing it to enlarge. A node is generally considered swollen when it exceeds 1 centimeter in diameter, though what counts as “normal” varies by location and age.
How Lymph Nodes Work as Filters
Lymph nodes are stationed throughout your body, clustered in areas like the neck, armpits, and groin. They act as checkpoints where immune cells screen the fluid draining from nearby tissues. When a threat is detected, specialized cells inside the node begin multiplying and coordinating an attack. This rapid buildup of immune cells is what physically enlarges the node, sometimes enough that you can feel it through the skin.
The swelling is localized. Nodes near the site of an infection or injury are the ones that react, which is why the location of a swollen node often points directly to the source of the problem.
Infections: The Most Common Cause
The vast majority of swollen lymph nodes are caused by ordinary infections. A cold, flu, or sore throat will commonly enlarge the nodes along your neck and under your jaw. An infected cut on your hand can cause swelling in the armpit on the same side. A sexually transmitted infection or a skin infection on the leg can trigger groin nodes.
Bacterial infections are frequent culprits, particularly strep and staph bacteria. These can cause not just node enlargement but active infection of the node itself, a condition called lymphadenitis, which may involve redness, warmth, and tenderness over the swollen area. Cat scratch disease, caused by Bartonella bacteria, is another well-known trigger, typically enlarging nodes near the scratch site.
Viral infections tend to cause more widespread swelling. Mononucleosis (caused by Epstein-Barr virus), HIV, and measles can enlarge nodes in multiple locations at once. Fungal infections are less common but follow the same pattern: the nodes nearest the infection site swell as they work to contain the threat.
What the Location Tells You
Because lymph nodes drain specific regions of the body, where you feel the swelling narrows down the likely cause.
- Neck and jaw: Upper respiratory infections, strep throat, ear infections, dental infections, and mononucleosis are the usual suspects. These nodes filter fluid from the head, mouth, and throat.
- Armpits: The armpit contains several groups of nodes that drain the upper limb, chest wall, and breast tissue. Swelling here can follow an arm infection, a skin wound, or breast-related conditions. Breast cancer sometimes first shows up as a painless, enlarged armpit node.
- Groin: Nodes here drain the legs, lower abdomen, and genital area. Skin infections on the feet or legs, sexually transmitted infections, and cellulitis are common triggers.
- Above the collarbone: Swelling in this area is less common and more likely to need investigation, as it can signal problems in the chest or abdomen.
Swelling in a single area (localized) usually points to a nearby infection or injury. Swelling in multiple areas at once (generalized) is more typical of systemic infections, autoimmune conditions, or blood cancers.
Autoimmune Diseases
When the immune system mistakenly attacks the body’s own tissues, lymph nodes can swell as part of that overactive response. Rheumatoid arthritis, lupus, and inflammatory bowel disease all involve chronic immune activation that can keep nodes enlarged for weeks or months. The swelling tends to come and go with disease flares and may appear in multiple locations.
Cancers That Affect Lymph Nodes
Lymphoma, a cancer that starts in the lymphatic system itself, is one of the most well-known causes of persistent node swelling. The hallmark is painless, firm enlargement, often in the neck, armpits, or groin. Other symptoms can include unexplained weight loss, drenching night sweats, and persistent fatigue. Leukemia, particularly chronic lymphocytic leukemia, can also cause widespread node enlargement.
Cancers that originate elsewhere in the body can spread to nearby lymph nodes. Breast cancer frequently metastasizes to armpit nodes. Lung and stomach cancers may enlarge nodes above the collarbone. In these cases, the swollen node contains cancer cells that have traveled through the lymphatic system from the original tumor.
Cancer-related swelling differs from infection-related swelling in a few important ways. The nodes tend to be painless, hard or rubbery, and fixed in place rather than movable under the skin. They grow over weeks without shrinking back down.
Medications and Vaccines
Certain medications can trigger lymph node swelling as a side effect. Anticonvulsants like phenytoin and carbamazepine are among the most common culprits, along with allopurinol (used for gout), minocycline, and dapsone. These drugs can cause a hypersensitivity reaction that mimics lymphoma on imaging, sometimes leading to unnecessary biopsies before the medication is identified as the cause.
Vaccines can also enlarge nearby nodes. After COVID-19 vaccination, armpit nodes on the injection side commonly swelled. Most cases resolved within 4 to 6 weeks, though in some individuals it took several months for nodes to return to normal size. This is a normal immune response, not a sign of a problem.
How Long Swelling Typically Lasts
Nodes that swell from a routine infection usually begin shrinking within one to two weeks as the infection clears. However, they don’t always snap back immediately. It’s common for a node to remain slightly enlarged for several weeks after you feel better. In children, whose immune systems are more reactive, nodes up to 2 centimeters can be normal in some situations.
Swelling that persists beyond four to six weeks without an obvious cause, continues to grow, or is accompanied by unexplained weight loss, night sweats, or fever warrants medical evaluation. Nodes that feel hard, fixed to surrounding tissue, or painless are more concerning than soft, movable, tender ones. A node above the collarbone at any size deserves prompt attention, as this location has a higher association with serious underlying conditions.
Less Common Triggers
Tuberculosis remains a significant cause of lymph node swelling worldwide, particularly in the neck. The nodes may cluster together and occasionally drain through the skin. Some rare infections, including toxoplasmosis (often from undercooked meat or cat feces) and certain fungal infections like histoplasmosis, can also cause enlargement.
Conditions like sarcoidosis, where clusters of inflammatory cells form in various organs, frequently involve the lymph nodes. Thyroid disorders, particularly Graves’ disease, can enlarge neck nodes. Even significant physical stress or a vigorous immune response to a minor injury can occasionally cause noticeable swelling in the nearest node group.