| The hallmark of acne vulgaris is the comedo, commonly known as the blackhead. |
| A closed comedo, or white head, occurs when the blockage is complete. |
| An open comedo is known as a blackhead, and a closed comedo is commonly referred to as a whitehead. |
| If the plug enlarges and is on the surface of the skin, exposed to the air, it is called an open comedo, or blackhead. |
| Not only do retinoids inhibit comedo formation, they can usually clear all degrees of comedonal acne, but this may take a few months. |
| If the plugged follicle stays below the surface of the skin, the lesion is called a closed comedo. |
