A 42-year-old man with a 6-month history of bilateral conjunctival injection, mild mucoid discharge, and recurrent episodes of exacerbation and remission is evaluated. Examination reveals giant papillae on the upper tarsal conjunctiva and a gelatinous appearance of the limbal conjunctiva. A 38-year-old woman presents with acute bilateral conjunctivitis, severe itching, and chemosis that worsens in spring and summer. Examination shows giant papillae on the upper tarsal conjunctiva and a milky-white appearance at the limbus. Which finding best distinguishes vernal keratoconjunctivitis from allergic conjunctivitis?