About 45 million people in Europe have allergic rhinitis (hay fever) - inflammation of the nasal passages causing sneezing, runny nose, nasal congestion, itching and tearing of the eyes. In the United Kingdom (UK), seasonal hay fever due to grass pollen allergy accounts for approximately 7 times more doctors' appointments than asthma. The standard treatment for hay fever consists of treating the symptoms with a nasal spray and an antihistamine. However, in a survey taken in a UK general practice less than 40% of patients with hay fever reported good symptom control with this standard treatment. For those patients with hay fever whose symptoms are not well controlled by treatment with antihistamines and nasal sprays, subcutaneous immunotherapy (SCIT) - (monthly injections of a grass allergen extract for a period of 3-5 years) is an effective alternative, and is approved in the UK on a named patient basis. More recently, allergen immunotherapy tablets (AITs) have been developed, including grass pollen allergen tablets. These have been shown to be highly effective in the treatment of hay fever, with the additional benefit of being convenient for patients, given that they may be taken at home. Grazax® (manufactured by Allergologisk Laboratorium København (ALK)-Abello, Denmark) has UK and European Union (EU) license for use in the treatment of troublesome grass pollen induced hay fever. The aim of this research is to investigate the effects of the AIT treatment on the immune system over time - which changes are taking place and when in the course of treatment. This will provide insight into the complexities of the development of allergen-specific immune tolerance - how harmful allergic responses against innocuous substances such as grass pollen can be overridden.