A whitehead, also known as a closed comedo, is a type of acne lesion. It occurs when a hair follicle becomes clogged with sebum (an oily substance produced by sebaceous glands), dead skin cells, and sometimes bacteria. Unlike blackheads, which are open to the air and oxidize to form a dark color, whiteheads remain closed, trapping the material inside and resulting in a white or flesh-colored bump on the skin.