In alopecia areata, the immune system breaks a hair follicle's special protection
Hair follicles normally enjoy a kind of diplomatic immunity: the body's defences leave them alone. In alopecia areata that shield fails, immune cells swarm the follicles, and coin-sized bald patches appear, often with no obvious trigger. About 2 percent of people experience it at some point in their lives.
The condition is thought to be autoimmune. Doctors call the follicle's protected status immune privilege, and in alopecia areata that privilege is breached. T lymphocytes gather around growing follicles, inflammation follows, and hair production slows or stops. The skin underneath stays unscarred and looks almost normal, which is why the hair can return.
Genes clearly matter. If one identical twin has it, the other has roughly a 50 percent chance of developing it too. Family studies point to at least four regions of the genome, and a 2010 genome-wide study linked 129 genetic variants to the disease, many involved in regulating T cells. It shares inherited risk with rheumatoid arthritis, type 1 diabetes and coeliac disease, and it is not contagious. Stress and illness may set it off in susceptible people, while smoking and obesity have been associated with higher risk.
The usual first sign is a round or oval bald patch on the scalp or beard, sometimes tingling or slightly sore. Around the edges doctors often find exclamation point hairs, only 3 to 4 millimetres long and thinner towards the root. A magnifying technique called trichoscopy can reveal yellow dots and black dots in the follicle openings. Nails may develop tiny pits.
Outcomes vary widely. In sudden patchy cases, hair regrows within a year in more than half of people, and in up to 80 percent of those with just one or two patches, though repeat episodes are common. When every body hair is lost, called alopecia universalis, fewer than 10 percent recover without treatment. There is no cure, though some treatments can speed regrowth. At any given moment about 0.15 percent of people are affected, onset usually comes in childhood, and females are affected more often than males.
Source: Alopecia areata
