Clinical meaning
Alopecia refers to partial or complete loss of hair from areas of the body where it normally grows. The hair follicle is a complex mini-organ embedded in the dermis that undergoes a continuous cycle of growth (anagen phase, lasting 2-7 years), regression (catagen phase, lasting 2-3 weeks), and rest (telogen phase, lasting approximately 3 months). At any given time, roughly 85-90% of scalp hairs are in the anagen phase. Normal hair loss is approximately 50-100 hairs per day. Alopecia areata is an autoimmune condition in which CD8+ T-lymphocytes attack the hair follicle bulb during the anagen phase, causing the hair to prematurely enter the catagen and telogen phases and fall out. The immune system specifically targets the hair follicle's immune privilege -- a mechanism that normally protects the follicle from immune attack. The result is well-circumscribed, smooth, round patches of complete hair loss without scarring or skin changes. Alopecia totalis involves complete scalp hair loss, while alopecia universalis involves loss of all body hair. Androgenetic alopecia (pattern baldness) is the most common form of hair loss, affecting both males and...