Doctors are unsure what causes alopecia areata, which originates in the immune system. Four years ago, the researchers learned that immune cells damage hair follicles and subsequently stop them from growing, but that ruxolitinib caused the follicles to thwart those immune cells. They also identified a group of T cells that actually does the attacking. In that study, two FDA-approved drugs — ruxolitinib, which treats a rare form of bone marrow disease, and tofacitinib, which is for rheumatoid arthritis — fully restored hair in mice within 12 weeks, and the new hair lasted months after treatment ceased.