The study will be conducted over 44 months. We expect to screen 70 atopic patients in order to enroll up to 50 suitable atopic participants to ensure randomisation of at least 40 atopic participants following their baseline visit. Additionally, we shall recruit 20 healthy, non-atopic volunteers. Individuals with moderate to severe grass pollen hay fever, with or without associated seasonal asthma, will be recruited and screened after the pollen season from September through March 2014. Atopic Participants will undergo baseline assessments in December 2013 to March 2014. All screening assessments will be completed before eligible participants are randomized to active or placebo treatment. Atopic participants will then begin AIT treatment in February-March 2014 and continue treatment for 12 months. All atopic participants will be provided with anti-allergic rescue medications (antihistamine tablets, topical intranasal corticosteroids, and eye-drops) throughout the pollen season. Clinical surrogate endpoint assessments and on specific time Points blood, nasal fluid and nasal brushing sampling, will be performed at baseline (January-March 2014), at 4, 8, 12 and 16 weeks after starting the AIT, and at 6 and at 12 months (January-March 2015) of treatment. Nasal mucosal biopsies will be taken at baseline, during the Peak pollen season, and 12 months of treatment. After 12 months of treatment, unblinding will take place. Those atopic participants receiving active AIT treatment will continue therapy for another 12 months followed by a withdrawal phase of 12 months. Blood samples and nasal biopsies will be taken from these participants again at 24 and at 36 months from the initial start of treatment. Those atopic participants on placebo will be offered 24 months of active treatment after unblinding.