Acne lesions are caused when a skin pore gets clogged, skin oil builds up inside the pore, and bacteria feeds on the skin oil and multiplies. All acne starts as a clogged pore, which can then develop into acne that falls into two categories: (1) non-inflammatory and (2) inflammatory. Non-inflammatory acne includes whiteheads and blackheads. Inflammatory acne includes papules, pustules, nodules, and cysts.
For reasons no one completely understands, these follicles sometimes overproduce cells and become blocked. Sebum (skin oil) which normally drains to the surface gets trapped and bacteria begins to grow.1, 2-11
All acne lesions start out as a microcomedone.1,12-13
Microcomedones can spontaneously become unplugged and heal or they become non-inflamed skin blemishes called comedones – either a whitehead or a blackhead.12-13
Whitehead. When the trapped sebum and bacteria stay below the skin surface, a whitehead is formed. Whiteheads may show up as tiny white spots or they may be so small that they are invisible to the naked eye.1,13
Blackhead. A blackhead occurs when the pore opens to the surface and the sebum, which contains the skin pigment melanin, oxidizes and turns a brown/black color. It is not dirt and cannot be washed away. Blackheads can last for a long time because the contents very slowly drain to the surface.1,13
A blackhead or whitehead can release its contents to the surface and heal or the follicle wall can rupture and inflammatory acne can ensue. This rupture can be caused by random occurrence or by picking or touching the skin. This is why it is important to leave acne-prone skin relatively untouched.1,12-13
Papule. A papule occurs when there is a break in the follicular wall. White blood cells rush in and the pore becomes inflamed.13
Pustule. A pustule forms several days later when white blood cells make their way to the surface of the skin. This is what people usually refer to as a "zit" or a "pimple."13
Nodule. When a follicle breaks along the bottom, total collapse can occur, causing a large, inflamed bump that can be sore to the touch.13
Cyst. Sometimes a severe inflammatory reaction can result in very large pus-filled lesions.1,13
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