Evaluation of the efficacy of sodium cromolyn preservative-free ophthalmic solution compared with placebo in the prevention of symptoms of allergic conjunctivitis induced by Conjunctival Allergen Challenge (CAC) (see Appendix II) in patients with a known history of allergic conjunctivitis. This will be assessed through the primary endpoint, which is the itching subject’s assessment by a 5-point scale, 5 minutes after the CAC.