Comedone
A comedone is a blocked hair follicle without significant inflammation. Open comedones reach the surface and darken through oxidation, giving blackheads their colour, while closed comedones remain under a thin covering of skin as small pale bumps.
The blockage comes first, before any redness. A comedone is what a follicle becomes when its channel fills with sebum and cells that have not shed properly, and at that stage the immune system has not yet responded.
Two forms are described. An open comedone reaches the surface, where its contents oxidise on contact with air and darken. That darkening is the whole explanation for a blackhead: the colour is oxidation, not dirt, which is why scrubbing achieves so little. A closed comedone stays beneath a thin layer of skin and appears as a small pale bump.
Clinically the word does more work than it seems to. Comedones are characteristic of acne, and their presence or absence is one of the things a clinician looks for when distinguishing acne from conditions that resemble it. Rosacea can produce papules and pustules that look similar, but it does not produce comedones, and that single observation frequently settles the question.
The related word comedogenic describes a substance considered likely to contribute to blockage. It appears widely on packaging, where its meaning is looser than its clinical use suggests.
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