Acne occurs when the sebaceous glands attached to your hair follicles over produce an oily substance called sebum. In normal circumstances, sebum plays an important role in lubricating your hair and moisturising your skin. However, if over production occurs, the excess sebum mixes with dead skin cells and creates a plug in your tiny hair follicles. If the plug is close to the surface of your skin, this results in a whitehead. If the plugged follicle is open to the skin, the result is a blackhead.
To complicate matters, the normally harmless bacteria that live on your skin can contaminate and infect the blocked hair follicles causing more severe symptoms of acne such as papules, pustules, nodules or cysts.