A comedo is a clogged hair follicle (pore) in the skin. Keratin (skin debris) combines with oil to block the follicle. A comedo can be open (blackhead) or closed by skin (whitehead), and occur with or without acne.
Blackheads are caused by too much oil production of your sebaceous glands. This is why blackheads are generally seen on the oiliest parts of the face. Dead skin cells, sebum, and dirt can clog your pores. When sebum comes into contact with oxygen, it turns black.
Both blackheads and whiteheads, says Dr. Chwalek, occur “when a pore gets blocked with dead skin cells and oils.” The difference is that in the case of whiteheads the pore remains closed; whereas with blackheads, the pore is stretched open. … (That’s how blackheads earned their oh-so-pleasant name.)
Sebaceous filaments look like blackheads, but they are different. They can appear on the nose. … Sebaceous filaments are glands that channel the flow of sebum through the pores. Unlike blackheads, they are not a form of acne.