This study is designed to determine if treatment with abatacept is effective in decreasing allergic airway inflammation in mild, atopic asthmatics. Subjects will be recruited from the greater St Louis Metropolitan area. Eligible individuals will undergo a titrated skin prick test. Following baseline evaluation, fiberoptic bronchoscopy with segmental allergen challenge (SAC) will be performed. The subjects will be randomized to either placebo or abatacept. After 12 weeks of study drug, the subjects will undergo repeat SAC. The primary endpoint will be to determine if treatment with abatacept results in a 50% or greater decrease in the percentage of eosinophils recovered in the bronchoalveolar lavage (BAL) fluid following SAC as compared to placebo control. Secondary endpoints include measures of airway obstruction and hyperreactivity, airway inflammation and symptoms as well as determination of the safety of abatacept administration in this subject population.