Sebum is produced by sebaceous glands, which are distributed all over the skin except on the palms and soles of the feet. Sebum is secreted onto the skin and hair surface, where it functions as a protectant by creating a thin film on the surface of stratum corneum. Sebum is a complex mix of lipids and consists of triglycerides, wax esters, cholesterol esters, squalene as well as free cholesterol and fatty acids.
The overproduction of serum is usually associated with oily skin type. This condition is called seborrhea and it is usually linked to genetic factors but may also happen in subjects with non-greasy skin in specific stages of life such as adolescents, post-menopausal (due to hormonal changes), or it can also be triggered by stress or medication.