Sebum is an oily substance secreted by the sebaceous gland in mammalian bone. Its main function is to protect our hair and skin from dryness and make the skin and hair  waterproof. But if our skin has more sebum it probably cause acne and if sebum is too little, it will end yo with dryness and cracking in the skin. 
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Sebum is the protective oily secretion produced by the sebaceous glands present in our skin.
Sebum lubricates the skin surface and minimizes the microbial attack.
Sebaceous glands eliminate certain substances like sterols, hydrocarbons, waxes through sebum.
But over secretion of sebum may leads to intense skin problems such as Acne , Black heads etc.
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