Swollen lymph nodes often appear as tender bumps under the skin, and while they can be uncomfortable, they usually signal that the immune system is active.
What causes the swelling?
According to medical experts, the lymphatic system works like a network of one‑way channels that transport fluid filled with immune cells. As this fluid passes through the bean‑shaped nodes—about six hundred of them scattered from the knees to behind the ears—the cells look for viruses or bacteria. When a threat is detected, the nodes increase their production of disease‑fighting cells, which makes them enlarge. “When the job is done they return to their normal size,” noted Dr. Stanley Rockson, founding director of the Stanford Center for Lymphatic and Venous Disorders.
Typical locations where people notice the bumps include the neck, under the chin, the armpits and the groin. The nodes may feel rubbery and range from the size of a pea to a plum, sometimes tender, sometimes firm.
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Five common reasons for swollen lymph nodes
First, vaccines can trigger a temporary increase. A vaccine introduces a harmless piece of a pathogen, prompting the body to produce antibodies. “The vaccine creates antibodies in the lymph nodes. In order for the response to occur, those cells have to expand in number, which causes the nodes to enlarge,” the doctor explained. The swelling usually fades within a week and is most noticeable near the injection site.
Second, most infections lead to enlargement. Upper‑respiratory illnesses, strep throat, ear infections, measles, mononucleosis, skin infections, tonsillitis and conjunctivitis are among the culprits. Viral colds tend to resolve without treatment, while bacterial cases such as strep throat often need antibiotics. Rare infections like tuberculosis, toxoplasmosis or certain sexually transmitted diseases can also cause the nodes to swell, especially if multiple areas are affected.
Third, autoimmune diseases may be responsible. Conditions such as lupus or rheumatoid arthritis involve an overactive immune system that can swell the nodes, though not every autoimmune disorder presents this symptom. Doctors typically assess a broader set of signs—joint pain, fatigue or skin changes—to determine if an autoimmune process is at work.
Fourth, cancer is an uncommon but serious possibility. Less than one percent of people with swollen nodes have cancer, according to the American Cancer Society. In malignancies, the nodes may trap cancer cells, and some cancers originate within the lymphatic tissue itself, known as lymphoma. Persistent swelling that does not subside may warrant further investigation.
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Fifth, certain medications or drug reactions can cause enlargement, though this is less frequently reported. The reaction usually resolves after the offending agent is discontinued.
Most cases stem from routine infections or vaccine responses, mirroring trends seen in previous decades.
When to seek medical advice
Doctors recommend contacting a clinician if the nodes remain enlarged for more than two to four weeks, become increasingly painful, feel hard, keep growing, or are accompanied by night sweats, skin redness or unexplained weight loss. In children, any node larger than one centimeter should be evaluated. “It’s never wrong to get something evaluated when you’re concerned,” Dr. Rockson said. Early assessment can help determine whether the swelling is a normal immune response or a sign of a more serious condition.
