Administration of allergen-specific immunotherapy by the oral route, sublingual immunotherapy (SLIT), has been shown to be effective, with an improved safety profile compared with subcutaneous administration. However, the precise mechanisms underlying the induction of immune tolerance by SLIT remain unclear. Contact of the allergen with the antigen-presenting cells in oral mucosa is likely to be critical. Mucosal Langerhans cells can capture the allergen and transport it to local lymph nodes, which may favour the induction of T lymphocytes that suppress the allergic response. In addition, the production of blocking IgG4 antibodies and the involvement of mucosal B cells appear to play a role. There is a growing evidence to support the role of regulatory T cells in controlling the development of asthma and allergic disease. Nevertheless, there remains a lack of firm evidence that SLIT induces regulatory T cells, although preliminary in vitro data suggest that SLIT may increase interleukin-10, which has a clear role in suppressing the allergic immune response. Further studies are required to determine the involvement of regulatory T cells, the role of different dendritic cell subsets, mucosal B cells as well as the potential use of adjuvants during SLIT.