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blackhead is the common name for a type of comedo(ne), the characteristic feature of acne vulgaris. They occur mostly on the face, neck, and upper part of the back, because this is where sebaceous glands are most thickly distributed. These glands secrete sebum, a fatty lubricant, into the hair follices. The lining of the follicles, like the skin surface itself, continually sheds and renews its outermost layer, and this debris, along with the sebum, normally escapes onto the surface. When the lining fails to be shed properly the sebum accumulates below it in the hair follicle. The debris of skin cells, pushed outwards by the sebum contains the pigment melanin, and this accounts for the dark plug that closes off the pore. Inflammation may be caused by bacterial action and fatty acids formed in the sebum, resulting in papules (lumps) and pustules (‘boils’ or ‘plukes’). Male sex hormones enhance the activity of the sebaceous glands along with the stimulation of hair growth, accounting for their exuberant secretion, and hence the likelihood of acne, around the time of puberty, and for the fact that girls suffer less — though they are not entirely exempt.