Sometimes someone's immune system attacks the cells of the growing hair bulb. This reaction condition is termed Alopecia Areata.
Just as in Chemotherapy, hair follicles are forced into the catagen stage. Hairs break and fall out, typically in patches dispersed through the scalp.
isn't correlated to Alopecia Areata very serious condition referred to as Cicatricial Alopecia, within which the system attacks the Stem Cells within the knot of the follicle. This ends up in permanent hair loss.