Lymph nodes swell when immune cells rapidly accumulate inside them to mount a defense against something your body perceives as a threat. A normal lymph node is less than 1 cm across (roughly the size of a pea). When it’s fighting an infection, responding to inflammation, or reacting to another trigger, it can double or triple in size and become noticeable under your skin. The vast majority of the time, the cause is a common infection, and the swelling resolves on its own.
What Happens Inside a Swollen Node
Lymph nodes are small, bean-shaped hubs scattered throughout your body, connected by a network of vessels that carry fluid called lymph. They act as checkpoints where immune cells gather, scan for threats, and multiply before being sent out to fight. When your body detects a pathogen or other problem, white blood cells flood into the nearest nodes and essentially pile up there. That buildup creates internal pressure, which is what you feel as a firm, tender lump under your skin.
This swelling is a sign your immune system is working. The node stays enlarged until the threat is handled and the extra immune cells clear out, which typically takes a couple of weeks for a straightforward infection.
Infections: The Most Common Cause
Everyday viral and bacterial infections are responsible for the vast majority of swollen lymph nodes. A cold, flu, strep throat, or ear infection will usually cause swelling in the nodes closest to the infection site, most often in your neck. Mono (caused by Epstein-Barr virus) is a classic trigger for noticeably swollen neck nodes that can persist for weeks.
Skin infections, wounds, or even a cat scratch on your arm can cause swelling in the armpit nodes, since those nodes drain fluid from the upper extremities. Cat-scratch disease is one of the more well-known causes of armpit lymph node swelling. In the groin, sexually transmitted infections like herpes, syphilis, and chancroid are common culprits, along with skin infections on the legs or feet.
The pattern is consistent: nodes swell in the region closest to where the infection or injury is happening. That location is often your biggest clue to the cause.
Location Gives Important Clues
Your body has about 600 lymph nodes, but you can only feel a handful of them, mostly in your neck, armpits, and groin. Where the swelling shows up narrows down what’s going on.
- Neck: Upper respiratory infections, strep throat, ear infections, dental infections, mono. These are the nodes you’re most likely to notice during a common illness.
- Armpits: Infections or injuries of the arm and hand, cat-scratch disease, and reactions to vaccines given in the upper arm. Breast, lung, and skin cancers can also spread to the armpit nodes, though this is far less common.
- Groin: Lower extremity skin infections, sexually transmitted infections, and occasionally cancers of the skin or reproductive organs.
When nodes swell in just one area, the cause is usually localized, like a nearby infection. When nodes swell in multiple areas at once (neck, armpits, and groin simultaneously), the trigger is more likely something systemic affecting the whole body.
Autoimmune and Inflammatory Conditions
Swollen lymph nodes don’t always mean infection. In autoimmune diseases, your immune system mistakenly attacks your own tissues, and the same flood of white blood cells that fights off a virus will build up in your nodes during a flare. Rheumatoid arthritis, lupus, and Still’s disease all commonly cause lymph node swelling, often in multiple areas at once.
Sarcoidosis, a condition where clusters of inflammatory cells form in various organs, can also cause persistent node enlargement. Less common conditions like Castleman disease and Kikuchi-Fujimoto disease affect the lymph nodes directly and can cause swelling that lasts weeks or months.
Vaccines Can Cause Temporary Swelling
Swollen lymph nodes after a vaccination are a normal immune response, not a sign of illness. The nodes in your armpit or neck on the same side as the injection may enlarge as your immune system processes the vaccine and builds protection. This became widely recognized during COVID-19 vaccination campaigns, but it can happen after any vaccine.
This type of swelling typically resolves within six weeks. If you have imaging scheduled (like a mammogram or chest scan), providers often recommend scheduling it before vaccination or at least six weeks after, so reactive nodes don’t get flagged as something concerning on the scan. If swelling from a vaccine persists beyond six weeks, it’s worth mentioning to your doctor.
Medications That Trigger Node Swelling
Certain medications can cause lymph nodes to enlarge as a side effect. Drugs that suppress or modify the immune system, like methotrexate (used for autoimmune conditions) and TNF-blocking medications used for inflammatory diseases, have been linked to lymph node changes ranging from general swelling to more significant immune cell buildup. If you’ve started a new medication and notice new swelling, the timing may be relevant.
When Swelling Points to Something Serious
Lymphomas and leukemias can cause lymph node swelling because cancerous immune cells multiply within the nodes themselves. Cancers from other parts of the body, like breast, lung, or skin cancers, can also spread to nearby lymph nodes. This is less common than infection-related swelling, but certain characteristics make it more concerning.
Nodes that are painless and firm or rubbery, nodes that keep growing over weeks, nodes that don’t go away after a typical infection clears, and swelling accompanied by unexplained weight loss, drenching night sweats, or persistent fevers all warrant medical evaluation. A node larger than 1 cm that persists beyond four to six weeks without an obvious cause is generally the threshold where further investigation is appropriate.
What Swollen Nodes Feel Like
Infected or reactive nodes are usually tender to the touch, somewhat soft or spongy, and move freely under the skin when you press on them. They often appear alongside other symptoms like a sore throat, runny nose, or fever, which helps point to the cause.
Nodes that feel rock-hard, don’t move when pushed, or are fixed to the skin or tissue underneath them feel different from a typical reactive node. Painless enlargement that comes on gradually, without any accompanying illness, has a different quality than the sore, swollen gland you’d get with the flu. Paying attention to these textures and patterns helps you gauge whether what you’re feeling is routine or worth getting checked.
How Long Swollen Nodes Typically Last
With a standard viral infection, most swollen nodes shrink back to normal within two to three weeks. Bacterial infections treated with antibiotics often resolve faster. Mono can keep nodes swollen for four to six weeks or occasionally longer.
Post-vaccine swelling follows a similar timeline, usually resolving within six weeks. Autoimmune-related swelling tends to come and go with disease flares rather than following a fixed timeline. The general rule is that nodes that haven’t returned to normal after six weeks deserve a closer look, especially if there’s no clear explanation for why they swelled in the first place.