In this case the pore is covered with a thin roof of skin. Comedones of both varieties constitute the mildest form of acne. Lesion is a generic term for any irritated acne skin bump, often used by dermatologists as a generic term for any abnormal skin manifestation. They result from rupture of sebaceous follicles. Papules are small bumps, often slightly red due to the surrounding inflammation.
Pustules are small, white-headed, inflamed bumps.
They contain pus and, as every acne-sufferer knows, when squeezed a small amount of white liquid can be expressed. Sebaceous follicles comprise the ducts and oil glands which become blocked, leading to the formation of acne papules, blackheads, and whiteheads. When ruptured they lead to more inflammatory lesions.