Allergen Immunotherapy via the subcutaneous or sublingual route is an effective treatment for allergic rhinitis but both are subject to limitations. Previous research has sought to further optimise allergen immunotherapy through a number of approaches including different routes of administration, such as epicutaneous or intralymphatic. Prior to this thesis work, a blinded study showed that repeated low dose grass pollen intradermal allergen injection suppressed allergen-induced cutaneous late phase responses comparably with conventional subcutaneous immunotherapy and more than with sublingual immunotherapy. In this thesis, the efficacy and safety of low dose grass pollen intradermal immunotherapy for the treatment of allergic rhinitis was evaluated in a randomised placebo-controlled trial with additional immunological analyses. Ninety-three adults with grass pollen-induced allergic rhinitis were randomly assigned to receive 7 pre-seasonal intradermal allergen injections (containing 7 ng of Phl p 5 major allergen) or a histamine control. The primary endpoint was daily combined symptom-medication scores during the 2013 pollen season (area under the curve). Analysis for the primary outcome was by intention-to-treat. Skin biopsy specimens were collected after intradermal allergen challenges, and late phase responses were measured 4 and 7, 10, or 13 months after treatment. Sera were collected for measurement of allergen-specific IgE and IgG. The results showed no significant difference in the primary endpoint between treatment arms (active, n=46; control, n=47; median difference, 14; 95% CI -172.5 to 215.1; p=0.80). Among secondary endpoints, nasal symptoms were worse in the intradermal immunotherapy group, based on daily (median difference, 35; 95% CI, 4.0-67.5; p=0.03) and visual analogue scale (median difference, 53; 95% CI, -11.6 to 125.2; p=0.05) scores. In a per-protocol analysis, intradermal immunotherapy was further associated with worse asthma symptoms and fewer symptom-free days. Intradermal immunotherapy increased serum Phleum pratense-specific IgE levels (p=0.001) compared with those in the placebo arm. T cells cultured from skin biopsy specimens of subjects undergoing intradermal immunotherapy had higher expression of the Th2 surface marker CRTH2 (p=0.04) and lower expression of the Th1 marker CXCR3 (p=0.01), respectively. No effect was seen on inflammatory cell numbers, including eosinophils, in skin biopsies collected after intradermal allergen injection as quantified by immunohistochemistry. Skin late phase responses nevertheless remained inhibited in the intradermal immunotherapy arm 7 months after treatment was completed (p=0.03). In conclusion, this randomised trial confirmed previous findings that repeated intradermal allergen injection suppresses skin late phase responses. However, this approach was not clinically effective as immunotherapy but resulted in worsening of respiratory allergic symptoms with some evidence for priming of type 2 responses in skin biopsy T cells.