A closed comedo is a whitehead, while anopen comedo is a blackhead. the plural of "comedo" is comedomes".
Blackheads are said to be the first stage ofacne. They form before bacteria invade the pores of the patient's skin. A blackhead can develop into a pimple, which is also known as a papule or pustule.
Blackheads, and acne in general, usually develop after puberty, when hormone levels surge and reach the skin. The presence of higher levels of hormones in the skin triggers the stimulation of the sebaceous glands, which produce oily substances. The sebaceous glands produce too much oil in the pores, which accumulates and gets stuck. When the occluded oil is exposed to air it becomes black.
Several conditions and circumstances can cause blackheads, or make them worse, such as the use of topical oils and make up. Blackheads can affect people with any type of skin, but are generally more common in those with oily skin. Source
