Recently interest has been focused on the administration of allergen specific immunotherapy by the oral route particularly sublingually. The mechanism by which sublingual immunotherapy exerts its effects remain unclear. Most likely, allergen captured within the oral mucosa by Langerhan's-like dendritic cells play a role in subsequent T cell responses. There is a growing body of evidence to support the role of regulatory T cells in controlling the development of allergic diseases. Nevertheless, there remains a lack or firm evidence that sublingual immunotherapy induces regulatory T cells. New vaccine developments with the increasing understanding of the molecular engineering techniques are on the way to offer the opportunity to design recombinant allergens that are safe, effective and easy to administer. In addition, the idea of using adjuvants along with allergen within the oral cavity is another promising approach.