This sebum is further converted to free fatty acids by bacterial lipases, specifically from the bacterium Propionibacterium acnes which is normally present on our skin (microflora). These free fatty acids, along with the presence of the bacteria cause inflammation of the skin, in a manner similar to an allergic response to a mousquito bite; however at a much larger degree. This leads to hyperkeratinization of the follicle (hair system) lining with subsequent plugging, leading to the appearance of papules (red) and comedones (black and white heads).
One important factor is an increase in hormones called androgens (male sex hormones). These increase in both boys and girls during puberty and cause the sebaceous glands to enlarge and make more sebum. Hormonal changes related to pregnancy or starting or stopping birth control pills can also cause acne.
Another factor is heredity or genetics. Researchers believe that the tendency to develop acne can be inherited from parents. For example, studies have shown that many school-age boys with acne have a family history of the disorder. Certain drugs, including androgens and lithium, are known to cause acne. Greasy cosmetics may alter the cells of the follicles and make them stick together, producing a plug.