Pimples contain oil, acne bacteria, dead skin cells and white blood cells. A pimple begins forming when sebum, an oily substance produced by the skin, becomes trapped in a pore. Bacteria begin to replicate in this sebum, leading to the formation of a pimple.
There are several different kinds of acne pimples. Blackheads form when the pore remains open and the sebum is exposed to air, turning it a dark color. Whiteheads form when the sebum becomes trapped beneath the surface of the skin but remains inside the pore. Blackheads appear as small, brown or black specs on the skin, while whiteheads appear as small, pin-head sized, white spots.
Larger, inflammatory acne lesions can form when the pore breaks internally, allowing the pimple contents to spill into the surrounding tissue. When this occurs, a large number of white blood cells rush to the area to fight off the bacteria, often leading to a large pocket of pus. This variety of an acne lesion is initially known as a papule, which develops a large, white head.
Very large inflammatory acne lesions, which sometimes involve several pores, are called cysts. A cyst generally contains a large collection of pus, and serious cysts must sometimes be lanced and drained to remove their contents.