Sebaceous cysts, (also known as epidermal inclusion cysts) are small round lumps that occur beneath the surface of the skin. They are typically caused by blockages that form in the sebaceous glands. The cysts occur as closed sacs that are filled with a white, pasty-looking protein called keratin mixed with sebum. If infected, the contents can be a mix of blood, keratin and other fluids. A cyst can sometimes have a central opening on the surface of the skin. It usually looks like a pore or hair follicle that has sunken deep into the skin. Although sebaceous cysts are generally harmless, they can become infected, causing swelling, redness and abscesses. It can be uncomfortable at sites of pressure.