In mice specially designed for testing drugs for the treatment of human alopecia, this medication worked to prevent the disease AA from starting. To test Abatacept, we are going to treat 60 patients with moderate to severe AA for 6 months. To make the study results meaningful, there will be a control or "placebo" group that does not receive the study drug. Patients will be randomly assigned to either receive the real or the inactive medication, and neither the patient nor the doctor will know which it is. The effectiveness of the medication will be measured by changes in hair re-growth as determined by physical exam and photography, as well as by patient and physician scoring. Patients will be followed for another 6 months off of the drug to see if the effects of treatment last and if there is delayed response. Small scalp biopsies and peripheral blood will be taken at the beginning of the study before treatment and then after 4,12 and 24 weeks. The chemical analysis of these skin samples and blood will help us to understand how the disease happens, how the treatment works, and even guide us to better treatments in the future.