Hair follicles are the tiny structures that grow hair in the scalp. Sebaceous glands produce sebum. On areas where acne develops, sebaceous glands surround the hair follicles. The combination of the sebaceous glands and the hair follicles is the "pilosebaceous unit," where acne pimples and cysts develop. Sebum moisturizes hair and skin. Each hair pushes up through the skin surface along with sebum.
Genetics play a big part in who gets acne and how severely, but each blemish can be blamed on some combination of sebum production, a bacteria called Propionibacterium acnes (P. acnes), plugged follicles, and inflammation. Finding a good treatment is really about finding the right combination of ingredients to troubleshoot each of those issues. Some factors that might worsen acne include hormones, certain medications, diet and stress.
Scientists initially hypothesized that acne represented a disease of the skin's hair follicle, and occurred due to blockage of the pore by sebum. During the 1880s, bacteria were observed by microscopy in skin samples affected by acne and were regarded as the causal agents of comedones, sebum production, and ultimately acne. During the mid-twentieth century, dermatologists realized that no single hypothesized factor (sebum, bacteria, or excess keratin) could completely explain the disease. This led to the current understanding that acne could be explained by a sequence of related events, beginning with blockage of the skin follicle by excessive dead skin cells, followed by bacterial invasion of the hair follicle pore, changes in sebum production, and inflammation.
Acne treatment that you apply to the skin: Most acne treatments are applied to the skin. Your dermatologist may call this topical treatment. There are many topical acne treatments. Some topicals help kill the bacteria. Others work on reducing the oil. The topical medicine may contain a retinoid, prescription-strength benzoyl peroxide, antibiotic, or even salicylic acid. Your dermatologist will determine what you need.