Acne can cause:
Blackheads and whiteheads (comedones). Comedones are enlarged hair follicles filled with sebum. Blackheads are comedones that have pushed through the skin's surface. Exposure to air causes the sebum to turn black. Whiteheads are comedones that have not pushed through the skin's surface.
Pimples (pustules). These are inflamed hair follicles. The bacteria in the follicle multiply, attracting infection-fighting cells. These release substances that cause irritation and redness. The follicle then ruptures and spills the contents into the surrounding skin. This causes more inflammation.
Nodules and cysts. These are larger infections of the hair follicles. They extend deeper into the skin, forming firm, deep bumps and swellings. Like pimples, they are caused by increased sebum production and bacterial growth, which cause irritation and redness.
In girls and women, acne often flares up at certain points in the menstrual cycle.