Acne occurs when pores become clogged with dirt or skin cells, which does not allow the skin’s natural oils to sweat to the surface. This causes a bump on the skin known as a whitehead, blackhead, papule, pustule, nodule, or cyst. There are two main types of acne: non-inflammatory and inflammatory. Blackheads and whiteheads are non-inflammatory, and pimples, pustules, nodules, and cysts are inflammatory. Inflammatory acne can be painful or itchy to the touch. In contrast, non-inflammatory acne is typically pain-free, though sometimes still itchy.
Non-inflammatory acne starts with microcomedone. Sometimes the body produces extra skin cells, which clog the pores and cause oils and bacteria to block the follicle. Doctors are still not sure why these skin cells are sometimes overproduced. At this point, a microcomedone appears and will become either a blackhead or a whitehead.
A whitehead is formed when this trapped bacteria and oil stay below the skin's surface. Whiteheads, as apparent in their name, appear as tiny white bumps. Sometimes they are so small that they are nearly invisible to the naked eye. Blackheads are formed when the bacteria and oil break the skin's surface and cause a black or brown bump to appear. Whiteheads clear up more quickly than blackheads because blackheads drain to the skin surface much more slowly than whiteheads.
When the follicle wall breaks, white blood cells rush to the site to repair it. This causes the pore to become inflamed and form a papule. If the follicle wall breaks, the white blood cells will clog the follicle and cause oils and bacteria to be stuck in the pore until the follicle wall is repaired. Papules are tiny and usually go unnoticed. A few days after the follicle wall breaks, the white blood cells will make their way to the skin's surface. This will cause the skin to become more raised and red and cause pustules. People typically refer to pustules as a 'zit' or a 'pimple.'
When the follicle wall breaks at the bottom of the pore, it can sometimes cause the pore's collapse. This will cause either a nodule or a cyst. A nodule is a large, inflamed bump that can be quite sore to the touch, while a cyst is a sizeable pus-filled lesion. Both are very annoying and can occur anywhere on the body.
Another type of acne is called milia. These are tiny white bumps caused when dead skin cells get stuck in small pockets on the skin's surface. It commonly occurs across the nose and upper cheeks. As more skin cells die and flake away, milia typically resolve itself. Still, the proper acne medication can help to speed the healing process.