IgE mediated Type I allergies represent a major health concern in the industrialized countries as they affect almost 25% of the population. Sensitized atopic individuals on re-exposure to an allergen suffer from rhinoconjunctivitis, dermatitis, bronchial asthma and life threatening anaphylactic shock. The only causative treatment for type I allergies is specific immunotherapy. This Ph.D thesis deals with the structural characterization of major group 3 allergen Phl p 3. With the help of comparison of structural features to related cross-reactive allergens and non cross-reactive allergens, structural epitopes have been delineated. In this work of in-silico prediction a novel strategy of including immunological data has been employed to increase the accuracy of the predictions. Based on these strategies two conformational epitopes having solvent accessible areas in range of 500 Å2 have been identified. Key residues have been identified in these epitopes for point mutations. Intact Phl p 3 with these point mutations will act as hypoallergenic derivative for specific immunotherapy. Non-merohedral twinning was observed during crystal growth, and subsequent diffraction patterns also indicated the presence of a second lattice in the crystal. Numerous crystals have been screened and data collection was performed to obtain a data set with no twinning effects. In addition to the twinning, pseudo-translation was observed along the c-axis. A high resolution data set (1.8 Å) lead to the satisfactory refinement due to the high amount of unique data generated at this resolution. We have observed gross deviations among pseudo-translated molecules, which had to modelled independently.