Acne occurs when glands (called sebaceous glands) in the follicles of the skin become overactive. These glands produce sebum, an oily substance that helps to stop the skin drying out. Someone with acne produces too much sebum, which forms a plug with dead skin cells and blocks the follicle.
If the blocked follicle is close to the skin surface, it balloons outwards and forms a whitehead. If the plug it the top of the follicle – opening on to the skin surface, it can be seen as a blackhead.
Bacteria that live on the skin – and which are usually completely harmless – can then infect the plugged follicles. This can cause the surrounding skin to become red and sore, and produce pus, leading to spots, nodules and cysts.
Areas of skin with the most follicles are the face, upper back and chest, which explains why acne tends to occur in these regions.