Human skin, especially on the face, neck, back, and chest is covered in hundreds of thousands of microscopic hair follicles, often called pores.
For reasons no one completely understands, these follicles sometimes overproduce cells and become blocked. Sebum (oil) which normally drains to the surface gets trapped and bacteria begins to grow.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
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
A papule or pustule can completely collapse or explode, severely inflaming the surrounding skin, and may engulf neighboring follicles. These lesions are called cysts or nodules:1,13
The following severe forms of acne are rare, but they inflict great hardship to the people who experience them.
Diagrams courtesy of: National Institute of Health