Immune system control of hair growth

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Hair growth regulation by the immune system

Experiments have established that distinct change in the number, location and activation of mast cells, macrophages, Langerhans cells and T cells can be observed during synchronized cycling in rats. This confirms the link between the immune system and hair growth cycle. ICAM-1, an adhesion molecule, which is expressed by some follicle compartments leads to the accumulation of perifollicular macrophages. This also suggests that hair cycle control gets affected by the changes in the number, location or activity of these macrophages.

Severe forms of hair loss or transformation from terminal hair to vellus hair sets in as a result of hair follicle bulb or the bulge isthmus region’s getting attacked by cell infiltration. These components of the skin immune system can therefore be said to control hair growth. It has been proved that immunosuppressive drugs like cyclosporine, FK506 and glucocorticosteroids control hair growth. Some of the hair growth regulating agents also have properties that influence the immune system. These are the drugs that act first and foremost directly on the hair follicle keratinocytes. Additionally they may also promote hair growth through altering the immune cells in the skin.

Research is underway to confirm the actual influence of the immune system on hair cycle control - especially the role of perifollicular mast cells and macrophages. Perifollicular mast cells are responsible for the remodeling of the skin tissue during hair cycling. Studies carried on the mouse skin have suggested that mast cells degranulate during anagen to catagen as well as the telogen to anagen transformation stage. So by blocking mast cell degranulation by inhibitors it is possible to block anagen and catagen development in hair follicles. While mast cell chemical secretions can induce catagen and anagen stages in mice, anagen development in vivo can also be slowed down through the administration of histamine or serotonin receptors.

Similarly, activated macrophages in the vicinity of the hair are found in the anagen to catagen stage in rats. Westgate postulated that during the end of anagen IV stage, there is an attack of macrophages on MHC class 1 negative keratinocytes of the hair bulbs. This it is believed triggers the onset of catagen. This theory however still lacks evidence. It is also notable that IL-1, TNF-alpha and FGF5, which are needed to induce catagen are all secreted by macrophages, suggesting a relation between the two.

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