Dried plugs of fatty material in the ducts of the sebaceous (oil-secreting) glands. Also known as comedowns. The black color at the surface of the plugs is not dirt, as often believed; albinos (who lack the pigment melanin) have light-colored "blackheads."

The blackhead is produced by faulty functioning of the sebaceous follicle as it releases sebum, the fatty secretion of the sebaceous glands. The consequent obstruction to the gland leads to enlargement of the follicle's opening. Infection by bacteria living on the skin may lead to inflammation and so to the characteristic papules, or "pimples," of acne.