The most common cause of inflamed lymph nodes is infection, particularly viral infections like the common cold, flu, or mononucleosis. When your body detects a threat, lymph nodes ramp up their immune activity and physically swell as a result. In most cases, the swelling resolves on its own within a few weeks. Less commonly, inflamed lymph nodes signal something more serious like an autoimmune condition, a medication reaction, or cancer.
Why Lymph Nodes Swell
Lymph nodes are small, bean-shaped structures scattered throughout your body that act as filters for your immune system. Each one contains specialized white blood cells ready to respond when they encounter something foreign. When a threat shows up, a handful of activated immune cells migrate into the center of the node and begin multiplying rapidly. Within about three days, these cells form organized clusters called germinal centers, where they produce antibodies tailored to fight the specific invader.
This rapid multiplication of immune cells, combined with increased blood flow and fluid accumulation, is what makes the node swell and feel tender. The node is essentially working overtime. You’re feeling your immune system doing its job.
Common Infections That Trigger Swelling
Viral infections are the single most frequent cause. A cold, the flu, or an ear infection can easily produce swollen nodes in the neck. Strep throat, mononucleosis, measles, and skin infections like cellulitis are other common culprits. An abscessed tooth can cause noticeable swelling in the lymph nodes under the jaw.
Some infections cause swelling in just one area, while others trigger it throughout the body. HIV and mononucleosis, for example, often cause generalized lymph node swelling in multiple locations at once. Localized swelling typically points to an infection near that specific group of nodes: a throat infection swells neck nodes, a hand wound swells armpit nodes, and a foot infection swells groin nodes.
Less common infections also belong on the list. Tuberculosis, syphilis, and toxoplasmosis (a parasitic infection from undercooked meat or contact with infected cat feces) can all cause lymph node swelling. Cat scratch fever, caused by bacteria transmitted through a cat scratch or bite, is a classic cause of swollen nodes near the injury site.
Non-Infectious Causes
Autoimmune diseases can inflame lymph nodes because the immune system is chronically activated, even without an infection to fight. Lupus is one of the more well-known examples, with swollen lymph nodes listed among its common symptoms. Rheumatoid arthritis and other autoimmune conditions can produce similar effects.
Certain medications trigger a drug hypersensitivity reaction that includes lymph node swelling as part of a broader inflammatory response. The most common offenders are anticonvulsant drugs like carbamazepine, phenobarbital, and phenytoin, along with allopurinol (used for gout), olanzapine, and sulfonamide antibiotics. This reaction typically develops within the first few weeks of starting a new medication. Some drugs, like certain antibiotics and contrast dyes used in imaging, can cause it in as little as two weeks.
Cancers, particularly lymphomas and leukemias, can cause lymph node swelling because the nodes themselves become sites of abnormal cell growth. Cancers that start elsewhere in the body can also spread to nearby lymph nodes, causing them to enlarge.
What the Location Tells You
Where a lymph node swells often points directly to the cause. Neck nodes are the most commonly affected and usually reflect a nearby throat, ear, or respiratory infection. Armpit nodes may swell from a skin infection on the hand or arm, or after certain vaccinations. Groin nodes respond to infections or injuries in the legs, feet, or genital area.
One location carries special significance. Swelling above the collarbone, in the supraclavicular area, has the highest association with cancer of any lymph node site. In people over 40, an estimated 90% of supraclavicular swelling is linked to malignancy. Even in younger adults, that figure is around 25%. Nodes on the right side above the collarbone tend to be associated with cancers in the lungs or esophagus, while the left side drains from deeper in the chest and abdomen, potentially signaling problems in the stomach, kidneys, pancreas, or reproductive organs.
How Long Swelling Normally Lasts
Infection-related swelling typically peaks within the first week and gradually shrinks over two to four weeks as the infection clears. You may still feel the node for a while after other symptoms have resolved, which is normal. The general medical guideline is that newly swollen lymph nodes without other concerning symptoms can be safely monitored for up to four weeks before further investigation is needed.
If swelling persists beyond three to four weeks without a clear explanation, a biopsy or imaging may be recommended to rule out more serious causes. Nodes that continue to grow, feel hard or rubbery, are fixed in place rather than movable, or appear without any obvious infection warrant earlier evaluation.
Warning Signs Beyond Swelling
Swollen lymph nodes become more concerning when they appear alongside specific systemic symptoms. In the context of lymphoma, doctors look for a combination known as B symptoms: unexplained fevers above 100.4°F (38°C), drenching night sweats severe enough to require changing your bedclothes, and unexplained weight loss of more than 10% of your body weight over six months. Any one of these paired with persistent, painless lymph node swelling is a combination that warrants prompt medical attention.
Painless nodes are actually more concerning than tender ones. Tenderness usually means the node is reacting to a nearby infection, which is a normal immune response. A hard, painless, and growing node is more likely to reflect something that needs further workup. Nodes that are easily movable under the skin are generally more reassuring than those that feel anchored to surrounding tissue.