According to the American Academy of Dermatology, the four basic mechanisms contributing to acne are hormones, increased sebum production, changes inside hair follicles, and bacteria. Acne usually occurs at age 11 to 14 when the body starts producing male hormones called androgens. Androgens can over-stimulate sebaceous glands and make them produce more sebum.

Dead cells inside hair follicles normally are shed and come out onto the surface of the skin. But in people with acne, the cells are shed faster, stick together, mix with sebum, and clog the follicle. Then bacteria contaminate the skin cell and sebum mixture and grow. When the body’s immune system tries to destroy the bacteria, inflammation results.

Source: U.S. Food and Drug Administration