Dandruff and Seborrheic Dermatitis: A Head Scratcher 392 recent advances in the understanding of these factors, especially the role of the yeast Malassezia. Sebum The role of sebum in dandruff is implied by the strong temporal correlation with sebaceous gland activity. This correlation includes increased incidence during infancy (cradle cap), low incidence from infancy to puberty, an increase in adolescence and young adulthood, and a decrease later in life.17-19 In addition, dandruff occurs exclusively on skin in areas with high levels of sebum. The function of human sebum has been and remains controversial, but recent advances in analytical technology have made some progress possible. Sebum is involved in epidermal development and barrier maintenance,20 transporting antioxidants,21 protection, body odor, and generation of pheromones.22 Sebum is directly involved in hormonal signaling, epidermal differentiation, and protection from ultraviolet (UV) radiation.23-24 Human sebum is a complex mixture of triglycerides, fatty acids, wax esters, sterol esters, cholesterol, cholesterol esters, and squalene (Figure 4).16,25 When secreted, sebum consists of triglycerides and esters which are broken down by microbes into diglycerides, monoglycerides, and free fatty acids (Figure 5). The free fatty acids play a key role in initiation of the irritant response, which is involved in scalp hyperproliferation. The role of sebaceous secretion also Figure 3. A three-factor causal model for dandruff and seborrheic dermatitis