Final answer:
A closed comedo forms when the openings of the hair follicles are blocked with debris and white cells. It appears as a small, flesh-colored bump under the skin.
Step-by-step explanation:
When the openings of the hair follicles are blocked with debris and white cells, it forms a closed comedo. A closed comedo is a type of acne lesion that appears as a small, flesh-colored bump under the skin, often referred to as a whitehead. Unlike an open comedo, which is a blackhead, a closed comedo remains closed and does not have an opening to the surface of the skin.