A hair follicle filled with excessive keratin and sebum forms a:

A hair follicle filled with excessive keratin and sebum forms a comedo, which is commonly referred to as a blackhead or whitehead depending on whether it is open or closed. This condition arises when the hair follicle becomes clogged with dead skin cells—which are primarily composed of keratin—and an overproduction of sebum, the oil produced by sebaceous glands.

When the clogged material reaches the surface and turns dark due to oxidation, it's called a blackhead. Conversely, when the follicle remains closed underneath the skin, it appears as a whitehead. This process highlights the role of keratin and sebum in comedo formation, making it distinct from other skin lesions like pustules and lesions associated with conditions such as acne, which may not always conform to the characteristics of a comedo.