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If a pore gets clogged up and closes but bulges out from the skin, you're left with a whitehead. If a pore gets clogged up but stays open, the top surface can darken and you're left with a blackhead. Sometimes the wall of the pore opens, allowing sebum, bacteria, and dead skin cells to make their way under the skin — and you're left with a small, red bump called a pimple.
What is Acne?