Comedones are associated with the pilosebaceous unit, which includes a hair follicle and sebaceous gland. These units are mostly on the face, neck, upper chest, shoulders and back. Excess keratin combined with sebum can plug the opening of the follicle. This small plug is called a microcomedo. Androgens increase sebum (oil) production. If sebum continues to build up behind the plug, it can enlarge and form a visible comedo.
A comedo may be open to the air (“blackhead”) or closed by skin (“whitehead”). Being open to the air causes oxidization, which turns it black. Propionibacterium acnes is the suspected infectious agent in acne. It can proliferate in sebum and cause inflamed pustules (pimples) characteristic of acne. Nodules are inflamed, painful deep bumps under the skin.