Lymph nodes swell when your immune system is actively fighting something, most commonly an infection. You have hundreds of these small, bean-shaped glands throughout your body, and when one becomes enlarged, it usually means the nearby area is dealing with a virus, bacteria, or another source of inflammation. The vast majority of swollen lymph nodes are harmless and resolve on their own within a couple of weeks.
Why Lymph Nodes Swell
Lymph nodes are gathering points for immune cells. When your body detects an infection or other threat, immune cells rush to the nearest lymph node to coordinate a response. These cells essentially pile in, creating pressure and swelling that you can feel under the skin. The node may grow from its normal size (typically under 1 centimeter) to something noticeably tender and firm, sometimes as large as a grape or even a small cherry.
The location of the swelling usually tracks with whatever’s going on nearby. A throat infection causes swelling in the front of your neck. A skin wound on your arm can trigger a swollen node in your armpit. An infection in your leg or groin area shows up as a swollen inguinal node in your upper thigh. This is because each group of lymph nodes drains a specific region of the body.
Common Infections That Cause Swelling
Viral infections are the most frequent trigger. A common cold, the flu, or mononucleosis (caused by Epstein-Barr virus) can all produce swollen nodes, particularly in the neck. These typically appear alongside other familiar symptoms like sore throat, fatigue, or congestion, and they shrink back down as the infection clears.
Bacterial infections are the next most common cause. Strep throat is a classic example, often producing tender, swollen nodes along the front of the neck. Other bacterial triggers include:
- Ear infections
- Abscessed teeth
- Skin infections like cellulitis
- Wound infections from staph or strep bacteria
In each of these cases, treating the underlying infection resolves the lymph node swelling, though the nodes may stay slightly enlarged for a few weeks after you feel better.
Less Common Causes
Some infections are rarer but still worth knowing about. Cat scratch fever, caused by bacteria transmitted through a cat scratch or bite, can cause significant swelling in the node nearest the wound. Toxoplasmosis, a parasitic infection picked up from cat feces or undercooked meat, is another. Tuberculosis, syphilis, and HIV can all produce swollen lymph nodes, sometimes as one of the earliest signs of infection.
Autoimmune conditions can also be responsible. When the immune system mistakenly targets the body’s own tissues, lymph nodes can swell just as they would during an infection. Lupus and sarcoidosis are two well-known examples. There’s also a rare genetic condition called autoimmune lymphoproliferative syndrome (ALPS), where the body overproduces a type of white blood cell, leading to persistently enlarged nodes in the neck, armpits, and groin that last for months or longer.
Cancers, particularly lymphoma and leukemia, can cause lymph node swelling. Cancers that start elsewhere in the body can also spread to nearby lymph nodes. This is the possibility that worries most people who search for this topic, but it’s important to keep in perspective: the overwhelming majority of swollen lymph nodes are caused by infections.
Where You Feel Them Matters
The three places where people most commonly notice swollen lymph nodes are the neck, armpits, and groin. These are the clusters closest to the skin’s surface and easiest to feel. Neck nodes (cervical nodes) respond to infections in the head and throat. Armpit nodes (axillary nodes) drain the arms, chest wall, and breast tissue. Groin nodes (inguinal nodes) respond to issues in the legs, feet, and genital area.
Some locations raise more concern than others. A swollen node just above the collarbone (the supraclavicular area) is considered abnormal at any size and warrants prompt evaluation. The same applies to nodes near the elbow or behind the knee that are palpable. Neck, armpit, and groin nodes, by contrast, swell routinely in response to minor infections and are rarely a sign of anything serious on their own.
Size, Duration, and What to Watch For
Doctors generally consider a lymph node enlarged when it exceeds 1 centimeter, roughly the width of a pencil eraser. Nodes larger than 1.5 to 2 centimeters raise more suspicion for something beyond a simple infection, such as a granulomatous disease or malignancy. But size alone doesn’t tell the full story.
Duration is a better clue. Swelling that lasts less than two weeks is almost always caused by a passing infection. Swelling that persists beyond one month without an obvious explanation, especially outside the groin, is more likely to need investigation. On the other end, nodes that have been stable in size for over a year with no change are very unlikely to be cancerous.
Other features that matter: a node that feels hard or rubbery rather than soft, one that’s fixed in place rather than movable under the skin, painless swelling (infected nodes are usually tender), and swelling in multiple unrelated areas at the same time. Accompanying symptoms like unexplained weight loss, drenching night sweats, or persistent fevers also shift the picture.
How Swollen Nodes Are Evaluated
In most cases, no workup is needed. If you have a cold, sore throat, or obvious infection, swollen nodes are expected and will resolve with the illness. Your doctor can often determine the cause from a physical exam and your symptoms alone.
When swelling is unexplained or has concerning features, the first step is usually blood work to check for infection or immune system abnormalities. Ultrasound is a common next step because it’s quick, painless, and good at revealing the internal structure of a node. Healthy nodes have a characteristic oval shape with a bright fatty center. Suspicious nodes lose that shape, becoming rounder with thickened or uneven outer layers.
If imaging raises questions, a needle biopsy can sample cells from the node without surgery. In some cases, the entire node is removed for closer examination. This is most likely when a node has been enlarged for over a month, is larger than 2 centimeters, or sits in a high-risk location like above the collarbone.
Swollen Nodes in Children
Children get swollen lymph nodes more frequently than adults, and the cause is almost always a routine infection. Kids’ immune systems are constantly encountering viruses and bacteria for the first time, which means their lymph nodes are frequently activated. It’s common for parents to notice small, pea-sized lumps along a child’s neck or jaw, especially during or just after a cold or ear infection.
Certain childhood infections are particularly associated with prominent lymph node swelling. These include mononucleosis, measles, rubella, and Kawasaki disease, an inflammatory condition that affects blood vessels in young children. Cat scratch fever also disproportionately affects kids, since they’re more likely to play roughly with cats and sustain scratches. The same general rules about size and duration apply, though doctors tend to be more conservative about biopsying nodes in children because reactive swelling is so common in this age group.