"The hair follicle is also attracting interest as a useful model system for studying a range of biological processes, including tissue development, epithelial cell differentiation, apoptosis (programmed cell death), and tumor formation. The immunology of the hair follicle, its relationship with the 'skin immune system' and its role in hair diseases remain biologically intriguing and clinically important. Understanding how hair loss might be prevented by inhibiting abnormal immune cell responses could lead to treatments not just for hair loss but also other important autoimmune diseases as well.
It is the research into the mechanism of androgen action in hair follicles and the pathogenesis of autoimmune disease of hair follicles that has given several observations that suggest that the immune system could be involved with hair growth and cycling.
Hair follicles provide numerous ports of entry into the body for micro-organisms.
The outward movement of the new hair shaft and displacement of the old club hair could be an avenue by which microorganisms can enter the more proximal hair follicle.
Hair growth is affected by substances with immunomodulatory characteristics, for example, cytokines, hormones, neuro-peptides, and some drugs.
Hair follicle regression during the catagen stage is associated with dramatic alterations in the peri-follicular populations of both macrophages and mast cells.
There is evidence that some autoimmune diseases damage the hair follicle.
On the other hand, there is a lack of MHC class I expression and Langerhans' cells and T cells in the proximal anagen hair bulb, suggesting this may be an immune privileged site. Gaining a better understanding of the immune privilege of the hair follicle may provide insights into the regulation and pathogenesis of immune-mediated diseases of the skin.
The hair follicle acts as a sensory organ and immunologic watchdog for the skin. Its complex immunologic profile of Langerhans cells, mast cells, lymphocytes and macrophages in the connective tissue, carry out surveillance for the immune system. Hairs detect mechanical stimuli above the surface of the skin, and the slightest bend in a hair activates neuroreceptors in the follicle, relaying important sensory information to the nervous system. The role of the Langerhans' cells at the opening of the follicle is to provide the immune system with enough information to prevent pathogens from invading the body through the skin. Langerhans' cells are present in large numbers in the infundibulum, but there are very few below the level of the sebaceous gland and almost none in the hair bulb. However, an important observation has been documented that they appear in the hair bulb during catagen, graying and in alopecia areata. The localization of Langerhan’s cells in the upper hair follicle suggests that they operate there as the key components of the ‘sentinel receptor pathway.’"