Lymph nodes swell when your immune system ramps up activity in response to a nearby threat, most commonly an infection. You have hundreds of these small, bean-shaped filtering stations throughout your body, and when one gets noticeably larger (typically over 1 centimeter), it usually means that node is working harder than usual to fight something off. The cause is almost always something straightforward, though in rare cases it can signal a more serious condition.
What Happens Inside a Swollen Node
Lymph nodes act as checkpoints where immune cells inspect fluid draining from nearby tissues. When those cells detect bacteria, viruses, or other threats, they multiply rapidly inside the node. This burst of immune cell production, combined with increased blood flow and fluid buildup, is what makes the node physically enlarge and sometimes feel tender. In an acute response, infection-fighting white blood cells flood the node first, followed by other immune cells that coordinate a longer-term defense.
The swelling itself is a sign the system is working. Think of it as a lymph node switching from standby mode to full production. Once the threat clears, the node gradually returns to its normal size, though this can take days to weeks depending on the cause.
Infections: The Most Common Cause
The vast majority of swollen lymph nodes are caused by infections, and the location of the swelling often tells you where the infection is. Nodes in your neck swell during throat infections, colds, ear infections, and dental problems. Nodes in your armpits respond to infections in your arms, chest, or breast tissue. Nodes in your groin react to infections in your legs, feet, or genital area.
Bacterial infections from Streptococcus and Staphylococcus are among the most frequent triggers, particularly skin infections, strep throat, and wound infections. Viral illnesses like the common cold, flu, mononucleosis, and HIV can also cause widespread node swelling, sometimes in multiple areas at once. Less common infectious causes include tuberculosis and cat scratch disease, a bacterial infection spread through a scratch or bite from an infected cat.
With a typical upper respiratory infection, you’ll notice tender, swollen nodes along the sides of your neck that peak in size over a few days and shrink as you recover. This kind of reactive swelling is the most common scenario and resolves on its own.
Vaccines Can Trigger Temporary Swelling
Vaccination is a frequently overlooked cause of lymph node swelling, and it caught widespread attention during COVID-19 vaccine rollouts. In clinical trials of the Moderna vaccine, about 12% of adults aged 18 to 64 reported armpit swelling and tenderness after the first dose, and 16% after the second dose. With the Pfizer vaccine, less than 1% of participants reported noticeable swelling, which typically appeared 2 to 4 days after vaccination and lasted about 10 days.
Interestingly, imaging studies tell a different story than what people feel. Ultrasound detected subclinical (not noticeable by touch) lymph node enlargement in nearly half of vaccinated patients scanned. This invisible swelling took longer to resolve, averaging about four months after the first dose. The swelling happens because the lymph nodes near the injection site are doing exactly what they should: mounting an immune response to the vaccine. It’s not a sign of a problem, but it can be alarming if you’re not expecting it, and it occasionally shows up on mammograms or other imaging as a false positive.
Autoimmune and Inflammatory Conditions
When the immune system mistakenly attacks the body’s own tissues, lymph nodes can swell chronically rather than in the short bursts seen with infections. Rheumatoid arthritis, lupus, and Still’s disease (a type of inflammatory arthritis) all commonly cause persistent node enlargement in the neck and other areas. Sarcoidosis, a condition where clusters of inflammatory cells form in various organs, is another well-known cause.
The key difference here is duration. Infection-related swelling comes and goes. Autoimmune-related swelling tends to persist or recur over weeks and months, often alongside other symptoms like joint pain, fatigue, fevers, or rashes.
Medications That Cause Node Swelling
Certain medications can trigger lymph node enlargement as a side effect. Seizure medications like phenytoin and carbamazepine are among the most commonly implicated. Other known culprits include allopurinol (used for gout), some blood pressure medications like captopril and atenolol, certain antibiotics including penicillin and cephalosporins, and sulfonamide drugs.
Drug-related swelling can appear in multiple node groups at once and may develop weeks after starting a new medication. If you notice unexplained swelling after beginning a prescription, that timing is worth mentioning to your doctor.
Cancer-Related Swelling
Cancer is a less common but serious cause of lymph node enlargement. Lymphomas (cancers of the lymphatic system itself) directly enlarge nodes. Other cancers, including breast, lung, and head and neck cancers, can spread to nearby nodes, causing them to swell.
Cancerous nodes tend to feel different from infection-related ones. They’re more likely to be firm or hard, fixed in place rather than freely movable under the skin, and painless. Benign, reactive nodes are typically soft, tender, and easy to move around with your fingers. Cancerous nodes also tend to grow steadily over weeks without shrinking, while infection-related nodes fluctuate and eventually resolve. On imaging, cancerous nodes lose their normal kidney-bean shape and become rounder, which reflects abnormal tissue replacing the node’s healthy internal structure.
None of these characteristics are absolute. A painless node isn’t automatically cancer, and a painful one isn’t automatically benign. But a node that’s been growing for more than two weeks without an obvious infection, measures over 2 centimeters, feels rock-hard, or is located just above the collarbone (a spot rarely involved in routine infections) warrants prompt evaluation.
Where You Feel the Swelling Matters
Your body has lymph node clusters in predictable locations, and each group filters fluid from specific regions:
- Neck (cervical nodes): drain the head, face, mouth, throat, and ears. Swelling here most often points to upper respiratory infections, dental infections, or throat infections.
- Armpits (axillary nodes): drain the arms, chest wall, and breast tissue. Swelling can follow arm injuries, skin infections, or vaccinations in that arm.
- Groin (inguinal nodes): drain the legs, feet, and genital area. Skin infections, sexually transmitted infections, and minor leg wounds are common triggers.
Swelling confined to one area (localized) usually points to a nearby cause. Swelling in multiple, unrelated node groups (generalized) suggests something systemic: a widespread viral infection, an autoimmune condition, or a medication reaction.
How Long Swelling Typically Lasts
For a routine infection, most swollen nodes peak within a few days and return to normal within two to four weeks. Nodes responding to a more significant infection like mononucleosis can stay enlarged for several weeks. Vaccine-related swelling usually resolves within 10 days if you can feel it, though imaging may detect subtle enlargement for up to four months.
Persistent enlargement beyond four to six weeks, progressive growth, or nodes that don’t fluctuate in size are patterns that typically prompt further investigation, which may include blood tests, imaging, or in some cases a biopsy where a small tissue sample is examined under a microscope. Most biopsied nodes turn out to be benign, but the evaluation rules out conditions that benefit from early treatment.