Lymph nodes swell because your immune system is ramping up to fight something, most commonly an infection. These small, bean-shaped structures are scattered throughout your body and act as filtering stations where immune cells gather, multiply, and launch their defense. When that happens, the node physically expands to accommodate the surge of activity inside it, and you feel it as a tender lump under your skin.
What Happens Inside a Swollen Node
Lymph nodes are part of your body’s drainage network. Fluid from nearby tissues flows through them, and immune cells inside scan that fluid for anything foreign: viruses, bacteria, damaged cells, or other threats. When they detect a problem, the node transforms into something closer to a command center.
White blood cells called lymphocytes rapidly divide inside the node, and the node itself must physically expand to make room. Immune cells called macrophages accumulate along the vessels inside the node and release growth signals that cause the node’s internal tissue to proliferate. B cells, another type of immune cell, also contribute by producing their own growth signals that enlarge the node further. This expansion is what you feel when you press on a swollen gland in your neck or under your arm. Once the threat is handled, a separate set of signals from T cells triggers the expanded tissue to shrink back down. The whole process is tightly regulated, which is why most swollen nodes return to normal size within a few weeks.
Infections: The Most Common Cause
The vast majority of swollen lymph nodes are caused by routine infections. A common cold, strep throat, or ear infection can trigger noticeable swelling in the neck within a day or two. An infected tooth does the same thing. These are the cases most people encounter, and they resolve once the infection clears.
Some infections cause more prolonged or widespread swelling. Mononucleosis (caused by Epstein-Barr virus) commonly swells nodes in the neck and can keep them enlarged for weeks. Skin infections like cellulitis, wound infections from staph or strep bacteria, and cat scratch fever (a bacterial infection from a cat bite or scratch) tend to swell the nodes closest to the affected area. Sexually transmitted infections including syphilis and herpes simplex typically cause swelling in the groin.
Certain infections cause lymph nodes to swell throughout the body rather than in just one area. HIV, measles, tuberculosis, and cytomegalovirus (CMV) can all produce this pattern of generalized swelling. If you notice lumps in multiple locations at once, that’s worth paying attention to.
Where the Swelling Appears Matters
Lymph nodes drain specific regions of the body, so the location of the swelling points toward the likely cause.
- Neck (cervical nodes) drain the head and neck. Swelling here is overwhelmingly caused by infections: sore throats, dental abscesses, ear infections, mono. Infection is far more common than anything serious in this location.
- Armpit (axillary nodes) drain the arms, upper chest, and sides of the breast. Infections or injuries to the arm are the most common trigger, along with cat scratch disease. However, axillary swelling carries a somewhat higher concern for malignancy than neck swelling, particularly when there’s no obvious infection or injury to explain it.
- Groin (inguinal nodes) drain the legs, lower abdomen, and genital area. Swelling here is usually benign and reactive. Sexually transmitted infections, lower leg skin infections, and minor injuries are the typical causes.
- Above the collarbone (supraclavicular nodes) are the exception to the “probably nothing serious” pattern. Studies have found that 34% to 50% of people with supraclavicular swelling had a malignancy, with risk highest in adults over 40. Swelling in this spot warrants prompt evaluation.
Autoimmune and Medication-Related Causes
Infections aren’t the only trigger. Several autoimmune conditions cause chronic or recurring lymph node swelling because the immune system stays persistently activated. Rheumatoid arthritis is a notable example: roughly 75% of people with RA develop swollen lymph nodes at some point, typically in the neck, above the collarbone, or in the armpits. Lupus frequently involves the lymph nodes and spleen as part of its widespread inflammation. Sarcoidosis, a condition that forms clusters of inflammatory cells in various organs, commonly affects lymph nodes as well and tends to appear between ages 20 and 39.
Certain medications can also cause lymph node swelling as a side effect. Drugs that suppress part of the immune system, including methotrexate (used for rheumatoid arthritis and psoriasis) and anti-TNF agents (used for conditions like Crohn’s disease, psoriasis, and ulcerative colitis), have been linked to abnormal lymph cell proliferation in the nodes. If you’ve recently started a new medication and notice new swelling, that connection is worth raising with your doctor.
Reactive Swelling vs. Something More Serious
Most swollen lymph nodes are “reactive,” meaning they’re responding normally to a nearby infection or irritation. But in a small percentage of cases, swelling signals something more concerning, like lymphoma or cancer that has spread from another site. The physical characteristics of the node itself offer useful clues.
Reactive (benign) nodes tend to be soft, oval or kidney-shaped, and tender to the touch. Their borders feel somewhat blurred rather than sharply defined. They move freely when you push on them. Pain is actually a reassuring sign in most cases, because it indicates active inflammation rather than tumor growth.
Nodes that raise more concern tend to be hard, round rather than oval, and painless. On imaging, they have sharp, well-defined borders and lack the normal internal structure (called a hilum) that healthy nodes show. They may feel fixed in place rather than sliding under your fingers. A single rock-hard, painless node that keeps growing over weeks is a different situation than a cluster of tender, squishy bumps during a cold.
How Long Is Too Long
Swollen nodes from a typical viral illness should start shrinking within two to three weeks and fully resolve within four to six weeks. Nodes that persist for several months without explanation can be caused by conditions like cat scratch disease, sarcoidosis, or atypical bacterial infections, but persistent swelling also overlaps with the timeline for lymphomas and other malignancies.
A few patterns deserve faster attention. Nodes that grow steadily larger over weeks without any signs of infection. Swelling in multiple unrelated areas of the body at the same time. Nodes that are hard, fixed, and painless. Unexplained weight loss, drenching night sweats, or persistent fevers alongside swollen nodes. And as noted above, any swelling above the collarbone. These don’t automatically mean cancer, but they shift the probability enough that imaging or a biopsy becomes worthwhile to sort out what’s happening.