On the skin, a comedo appears as a small mass which can be whiteish (closed comedo) or black (open comedo) when the part in contact with the air oxidizes and darkens.

It appears when the elimination of sebum , which normaly exudes from the skin’s sebaceous glands, is blocked by an obstruction (blocked hair follicle) or an occlusion at the epidermis.

The emergence and the development of comedos can be favoured by endogenous (oily skin…) or exogenous (the purpose of comedogenous products) factors.

In the case of bacterial infection, comedos can evolve into acne .