|MadSci Network: Immunology|
Sebum is secreted by the sebaceous glands (Fig. 1, from http://www.niams. nih.gov/hi/topics/acne/acne.htm and has several functions. As you point out, one of them is to reduce water loss from the skin, keeping us from shriveling up like prunes. Sebum also has weak antibacterial properties. Some of the saturated and unsaturated fatty acids found in sebum are bacteriostatic. They can inhibit the growth of bacteria, particularly gram positive organisms such as Staphylococcus aureus, and some fungi. Normally sebum empties onto the skin through our pores. If these become obstructed because of a combination of excess sebum production and accumulation of cells and debris, they can provide a fertile breeding ground for bacteria, notably Propionibacterium acnes leading to acne (Fig. 2-4 from the same source).
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