Less severe mild to moderate acne is caused by pores that get clogged with oil and dead skin and usually have a white or blackhead with slight inflammation. It becomes cystic acne when the pore shatters beneath the skin causing the inflammation to spill out into the area surrounding it. Your bodies defense is to create a cyst around it to stop bacteria and the inflammation spreading further.

Source: Health and Fitness: Acne Articles from EzineArticles.com