Specific immunotherapy or desensitisation is the practice of administering small amounts of allergen to allergic patients in increasing doses. This treatment is highly effective in these patients and furthermore is truly disease-modifying, with benefits persisting long-term, even when the treatment has been completed. Desensitisation is a routine treatment in the UK, Europe and North America. The exact immune mechanisms that underlie this symptomatic improvement are not entirely clear. Dr Tarzi, Professor Frew and Professor Kern have recently developed new methods for the investigation of immune responses to allergens. These methods require relatively small blood samples and may provide useful information about how immunotherapy exerts its effects. In addition to improving the investigators basic understanding of this treatment, such knowledge may drive improvements in the treatment and could be useful for monitoring patients for response. The investigators study proposes to investigate changes in the immune responses to pollen allergens during immunotherapy. Blood will be taken just prior to the first immunotherapy injection and again just prior to the final injection. In this way the investigators will be able to compare the immune responses to pollen allergen before and after treatment.