According to the Cleveland Clinic, it's unknown how common branchial cleft cysts (ones found on the side of the neck) are. "That's because not all branchial cleft cysts cause problems.

To my knowledge, this is the first probable second branchial cleft cyst we have found. However, it is possible there have been others that we did not recognize at the time.

A cyst is a sac-like pocket of tissue containing fluid or other substances. It can grow almost anywhere in the body or under skin. Discover 14 types of cysts.

A branchial cyst, a.k.a. a branchial cleft cyst, is a small, fluid-filled sac that an look like a lump under the skin on the side of your neck, according to the Cleveland Clinic.

Branchial cleft cysts are sacs filled with fluid found in the neck and can appear as lumps or swelling under the skin. They can range from one centimeter to 10 centimeters (almost four inches) in ...

That said, branchial cleft cysts are one of the most common forms of neck masses or lumps, especially in children," the clinic's website notes.

The cyst can also become infected and form an abscess, or a painful, puss-filled pocket. Once branchial cysts begin growing, doctors will advise immediate removal to prevent further infection.