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Acne refers to the appearance of blemishes, or small, inflamed bumps, on the surface of the skin. Acne typically appears on the face, chest, or back but can also appear in other places. It affects millions of people, regardless of age, ethnicity, skin type, and gender. 

All acne blemishes start the same way: Pores, which are tiny openings in the skin where hair emerges, become clogged with oil produced by glands connected to these pores. For this reason, people who have overactive oil glands may be more likely to have acne. Hormones can also cause excess oil production, which is why so many teenagers are affected. But acne can persist well into adulthood. For example, hormonal changes related to menstruation or menopause can cause acne in women, and stress, anxiety, or depression may lead to breakouts or make them worse. 

There are many types of acne blemishes, and just as many causes for their formation. Some develop simply because of oil trapped beneath the surface of the skin, whereas others form or worsen as a result of a bacterial infection in the pore. 

Blackheads

Blackheads are small blemishes that frequently appear in groups. They typically have a dark appearance that can be mistaken for dirt, but the color is actually due to oil that gets trapped in the pore. When this oil is exposed to air, it turns black.

Whiteheads

Whiteheads are similar in size to blackheads but have a rounder, lighter appearance. Whiteheads form because oil and dead skin cells clog the pore and cause it to close up at the surface of the skin.

Papules

Papules are small, red, inflamed blemishes that tend to cluster together—frequently on the forehead or cheeks—and can be sore to the touch. 

Pustules

If an inflamed blemish is filled with pus, it’s called a pustule. Pustules vary in size and are commonly known as pimples. Pus is a combination of oil, bacteria, and dead skin cells that gets trapped beneath the skin and accumulates, causing an infection. The body’s natural response is to fight the infection, but this immune response actually causes the redness and swelling that often surrounds these blemishes. 

Nodules

Nodules are bigger, swollen bumps that form when the walls of a hair follicle break down. As a result, bacteria and oil spread deeper into the skin. These bacteria lead to an infection that causes painful blemishes that may affect multiple pores. Because nodules are so deeply rooted, they can permanently damage skin tissue and result in scars. 

Cysts

Acne cysts are very large blemishes caused by a severe inflammatory reaction deep in the pore. Cysts form when a rupture in the internal wall of the pore allows bacteria and oil to spread into the surrounding skin, causing an infection. If a membrane forms around this infected area, it becomes a cyst. These blemishes are often painful.