Your lymph nodes may be small, but they’re mighty –  these tiny structures filter out everything from a cold to cancer.

But is it okay for your lymph nodes to be swollen?

Maybe, says Vipra Sharma, MD, MPH, an oncologist with Hartford HealthCare Cancer Institute.

“If you get a simple cold or more serious infection such as pneumonia – or even if you get something basic like poison ivy or dental infection – your lymph nodes work to protect your body,” Dr. Sharma says.

But sometimes the swelling can be a sign of something more serious. Dr. Sharma explains how you can tell the difference.

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Where are my lymph nodes?

Lymph nodes are small structures full of immune cells that use lymph fluid to attack and destroy foreign substances like viruses.

Lymph nodes can be found in:

  • Armpits
  • Chest
  • Abdomen
  • Groin
  • Neck

> Related: 6 Cancer Screenings That Could Save Your Life

What causes swollen lymph nodes?

A swollen lymph node can be caused by a simple infection or virus. For example, if the lymph nodes in the neck are enlarged, it could indicate infection of the throat or ears.

They also respond to systemic ailments like strep throat or even HIV or autoimmune diseases.

In rare cases, a swollen lymph node is a sign of lymphoma, which is cancer of the lymph system.

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How do I know when swollen lymph nodes are something more serious?

If your swollen lymph node lingers, it may be time to talk to your primary care provider.

“Since they generally indicate infection, most of the time they should get better in two to four weeks,” Dr. Sharma says.

Other warning signs to watch for include:

  1. Change in how the lymph node feels. “With a cold or flu, the node might feel enlarged and tender, but are freely movable,” she says. “If there’s a malignancy, the area will feel firm, hard, rubbery and fixed because there’s a collection of cancer cells there.”
  2. Change in size. Lymph nodes should feel like a little pea (less than a 1cm), Dr. Sharma notes. Let your doctor know if they grow larger, to the size of a peanut (2cm) or grape (3cm).
  3. Other symptoms like a fever, weight loss, night sweats or drainage from nearby sources like a nipple or ear.