['Allergic asthma is a T(H)2-promoted hyperreactivity with an immediate, IgE, and mast cell-dependent response followed by eosinophil-dominated inflammation and airway obstruction.', 'Because costimulation by CD28 is essential for T(H)2 but not T(H)1 responses, we investigated the effect of selective interference with this pathway in mice using the models of ovalbumin and house dust mite-induced airway inflammation.', 'To study the role of CD28 in the effector phase of allergic airway inflammation, we developed an inducibly CD28-deleting mouse strain or alternatively used a CD28 ligand-binding site-specific mouse anti-mouse mAb blocking CD28 engagement.', 'We show that even after systemic sensitization to the allergen, interruption of CD28-mediated costimulation is highly effective in preventing airway inflammation during challenge. In addition to improving airway resistance and histopathologic presentation and reducing inflammatory infiltrates, antibody treatment during allergen challenge resulted in a marked relative increase in regulatory T-cell numbers among the CD4 T-cell subset of the challenged lung.'] Does interruption of CD28-mediated costimulation during allergen challenge protect mice from allergic airway disease?