A cyst is a closed pocket or pouch of tissue. It can be filled with air, fluid, pus, or other material.
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Cysts may form within any tissue in the body. Most cysts in the lungs are filled with air. Cysts that form in the lymph system or kidneys are fluid-filled. Certain parasites, such as some types of roundworms and tapeworms, can form cysts within the muscles, liver, brain, lungs, and eyes.
Cysts are common on the skin. They can develop when acne causes a sebaceous gland to clog, or they can form around something that is stuck in the skin. These cysts are not cancer (benign) but can cause pain and changes in appearance. At times, they can become infected and need treatment due to pain and swelling.
Cysts can be drained or removed with surgery, depending on their type and location.
Sometimes, a cyst looks like a skin cancer and may need to be removed to be tested.
Linda J. Vorvick, MD, Clinical Associate Professor, Department of Family Medicine, UW Medicine, School of Medicine, University of Washington, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.