Comedo is a mass or plug of sebum and keratin inside the dilated opening of a follicle of hair. It frequently contains the bacteria Propionobacterium acnes, SPityrosporon ovale, and Staphylococcus albus.

There are two types of comedo – closed which are referred to as whiteheads. These occur when the opening is not dilated widely, and appear as a flesh-colored small papule. It might rupture causing a lesion in the dermis that is inflammatory.

The other type is an open comedo commonly referred to as a black head. This is a comedo with an opening that is widely dilated and the pigmented impaction is seen at the surface of the skin. It is usually the primary lesion of acne vulgaris.