Non-Inflamed Acne:   Dead skin cells and oil (plus other debris) form a plug inside the pore. If this plug does not become inflamed it can become a whitehead - a non-inflamed lesion under the skin, also called a "closed comedone.  Sometimnes called "maturation arrest" acne, as it has not "matured" into a blackhead yet.  

 

Inflamed Acne:  As the oil and the dead skin cells build up. they put pressure on the cells surrounding the pore.  Because this material contains a lot of P. acnes bacteria, the surrounding skin now becomes infected, creating a red bump that we know as a pimple.  The medical term for this red bump is an inflammatory papule.

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