Primary aim is to evaluate the efficacy of specific immunotherapy with Pangramin SLIT HDM-mix compared to placebo in the treatment of House Dust Mite (HDM) induced rhinitis with or without asthma. Sublingual immunotherapy has been used during several years and has been shown to provide benefits compare to traditional subcutaneous treatment. This study will investigate if improvements in rhinitis symptoms and less use of symptomatic medication can be obtained as a consequence of being treated under specific immunotherapy. This study aim also to contribute to the documentation of tolerability and safety profile of Pangramin HDM Mix.