<P> Sebum, secreted by the sebaceous gland in humans, is primarily composed of triglycerides (≈ 41%), wax esters (≈ 26%), squalene (≈ 12%), and free fatty acids (≈ 16%). The composition of sebum varies across species . Wax esters and squalene are unique to sebum and not produced as final products anywhere else in the body . Sapienic acid is a sebum fatty acid that is unique to humans, and is implicated in the development of acne . Sebum is odorless, but its breakdown by bacteria can produce strong odors . </P> <P> Sex steroids are known to affect the rate of sebum secretion; androgens such as testosterone have been shown to stimulate secretion, and estrogens have been shown to inhibit secretion . Dihydrotestosterone acts as the primary androgen in the prostate and in hair follicles . </P> <P> Sebaceous glands are part of the body's integumentary system and serve to protect the body against germs . Sebaceous glands secrete acids that form the acid mantle . This is a very fine, slightly acidic film on the surface of the skin that acts as a barrier to bacteria, viruses, and other potential contaminants that might penetrate the skin . The pH of the skin is between 4.5 and 6.2, and this acidity helps to neutralise the primarily alkaline nature of contaminants . </P> <P> Sebaceous lipids make an important contribution in maintaining the integrity of the skin barrier, and express both pro-inflammatory and anti-inflammatory properties . Sebum may act as a delivery system for antioxidants, antimicrobial lipids, pheromones, and hydration of the stratum corneum . The insoluble fatty acids contained within sebum have broad antimicrobial activity . Additionally, sebaceous gland secretion provides vitamin E to the upper layers of facial skin . </P>

The sebaceous glands are part of what system