The most common cause of swollen lymph nodes is infection, particularly viral infections like the common cold. Lymph nodes are small, bean-shaped structures scattered throughout your body that act as filters for your immune system. When your body detects a threat, these nodes ramp up production of immune cells, physically expanding in the process. While infections account for the vast majority of cases, swollen nodes can also signal autoimmune conditions, medication reactions, or, less commonly, cancer.
How Lymph Nodes Actually Swell
Lymph nodes contain dense clusters of immune cells organized into specific zones. When bacteria, viruses, or other foreign substances arrive via your lymph fluid, specialized zones within the node spring into action. One region rapidly produces B cells (the immune cells that make antibodies), while another generates T cells (which directly attack infected cells). This surge in cell production is what makes the node physically enlarge, sometimes to the size of a kidney bean or larger.
In some cases, the node itself becomes infected, a condition called lymphadenitis. This tends to be more painful and may cause the overlying skin to turn red and warm. The swelling you feel is essentially your immune system working overtime in a concentrated area.
Infections That Commonly Cause Swelling
Upper respiratory infections are by far the most frequent trigger. A cold, sore throat, or ear infection will often cause the lymph nodes in your neck to puff up. You might also notice tender nodes under your jaw from an abscessed tooth or along your neck during a bout of strep throat. These infections typically cause nodes that are tender, movable under your fingers, and clearly defined.
Some infections cause more widespread swelling across multiple areas of the body. Mononucleosis and HIV are two notable examples. Mono often produces noticeably swollen nodes in the neck along with extreme fatigue, while HIV can cause generalized swelling across several lymph node groups as the virus affects the immune system broadly.
Less common infections also deserve mention. Tuberculosis, syphilis, and toxoplasmosis (a parasitic infection from undercooked meat or contact with infected cat feces) can all trigger lymph node enlargement. Cat scratch fever, caused by a bacterial infection from a cat scratch or bite, typically swells the nodes closest to the wound.
Where the Swelling Appears Matters
Swollen lymph nodes near the site of a problem are your body’s way of pointing to the source. Nodes in your neck usually react to head and throat infections. Swollen nodes in your armpit might respond to an arm or breast infection. Groin nodes often enlarge from infections in your legs, feet, or genital area.
One location carries particular significance: the area just above your collarbone, called the supraclavicular region. Swelling here has the highest association with serious underlying disease. In people over 40, supraclavicular swelling is associated with malignancy in up to 90 percent of cases. Even in younger adults, about 25 percent of supraclavicular swelling turns out to be cancer-related. The left supraclavicular node specifically can signal problems in the chest or abdomen because it connects to the body’s main lymphatic drainage channel.
Autoimmune Conditions and Medications
Your immune system doesn’t only react to infections. In autoimmune diseases like lupus and rheumatoid arthritis, the immune system mistakenly attacks the body’s own tissues, and lymph nodes can swell as part of that misdirected response. The swelling tends to be generalized rather than limited to one area.
Certain medications can also trigger lymph node enlargement. Methotrexate, widely used for rheumatoid arthritis and psoriasis, can produce changes in lymph nodes ranging from mild swelling to more concerning overgrowth of immune cells. Some drugs used to suppress the immune system, including a class called TNF inhibitors, have been linked to similar lymph node reactions. Other medications can trigger a drug hypersensitivity syndrome, where the body mounts an exaggerated immune response that includes node swelling along with fever and rash. If you notice new lymph node swelling after starting a medication, that timing is worth mentioning to your doctor.
When Swelling Signals Cancer
Cancer causes lymph node swelling in two ways: cancer that starts in the lymph nodes themselves (lymphoma) or cancer that has spread from another part of the body. The physical characteristics of cancerous nodes differ noticeably from infection-related swelling.
Nodes swollen from a viral infection are typically soft, movable, and may be tender. Cancerous nodes tend to be hard, painless, and fixed in place, meaning they don’t slide easily under your fingers. Hodgkin lymphoma, for example, produces nodes that feel firm and rubbery. This contrast is one of the first things a clinician evaluates during a physical exam.
Several risk factors raise the likelihood that swollen nodes have a malignant cause:
- Age over 40
- Supraclavicular location
- Hard, fixed, painless nodes
- Swelling lasting more than four to six weeks
- Systemic symptoms like unexplained weight loss, drenching night sweats, or persistent fever
- Generalized swelling involving two or more separate regions of the body
That said, cancer remains a relatively uncommon cause of swollen lymph nodes overall. The vast majority of cases, especially in younger adults and children, turn out to be reactive swelling from an infection.
How Long Swollen Nodes Last
Most swollen lymph nodes return to their normal size once the underlying infection clears. For a typical cold or throat infection, this usually means days to a few weeks. There’s no precise universal timeline because it depends on the infection and your body’s immune response, but steady improvement is the key signal that things are resolving normally.
Swelling that lasts less than two weeks or persists longer than 12 months without changing size both have a low likelihood of being cancerous. The concerning window falls in between: nodes that keep growing or haven’t returned to their baseline size after eight to twelve weeks generally warrant further evaluation. If initial assessment suggests the swelling is likely benign and limited to one area, a four-week observation period is a reasonable first step before pursuing additional testing.
What Size Counts as Swollen
Not every node you can feel is abnormal. Lymph nodes in the neck are generally considered insignificant if they’re under 2 centimeters, roughly the width of a nickel. In the armpit and groin, the threshold is slightly higher at about 3 centimeters, since nodes in these areas tend to be naturally larger. The exception is the supraclavicular area, where any node over 1 centimeter is considered significant and worth investigating promptly.
Keep in mind that thin people or those with less soft tissue may be able to feel perfectly normal lymph nodes, especially in the neck and groin. A small, soft, mobile node that hasn’t changed in years is almost certainly nothing to worry about. What matters more than absolute size is whether a node is new, growing, or accompanied by other symptoms.