The primary objective of this study is to demonstrate the superiority of 2L®ALERG over placebo in terms of efficacy on the symptoms of allergic rhinitis and allergic rhinoconjunctivitis in patients with seasonal allergy to grass pollen, corrected according to rescue medication intake. The secondary objectives are to compare the allergy symptoms, the rescue medication intake, the quality of life and the safety in patients treated with 2L®ALERG or with a placebo.