A 28-year-old woman presents with acute onset conjunctivitis following an upper respiratory tract infection. Examination reveals papillary reaction of the tarsal conjunctiva with scanty mucoid discharge. A 35-year-old man with chronic allergic rhinitis presents with giant papillae (>1 mm) on the upper tarsal conjunctiva and thick ropy discharge. Which single feature best discriminates allergic conjunctivitis from viral conjunctivitis?