Acne is caused by the overproduction of sebum, or oil by the sebaceous glands of the skin, and further infection of glands by the bacteria. This can cause black head and comedones.
A little about the comedones
There are different kind of comedones.
•Open Comedones are the result of drained sebum papules, which later
on turns into pigmented skin due to over production melanin.
•Closed Comedones are the result of complete blockhead of glands
openings on to the skin surface. Due to the accumulation of sebum un-
derneath the skin, which results in the white heads on the skin.