Sublingual immunotherapy (SLIT) is an effective and safe treatment for respiratory allergy, and its mechanism of action currently is investigated with increasing attention. Studies of pharmacokinetics showed that allergen extracts administered via the sublingual route are not directly absorbed by the oral mucosa but are long retained at mucosal level, where the allergen molecules are captured by dendritic cells and, following their migration in the draining lymph nodes, presented to T cells. This seems to be the pivotal factor underlying the mechanisms of action of SLIT, at least for the long-term effects, and for the short-term efficacy, observed with ultrarush or coseasonal treatment, a down-regulation of mast cells resulting in hyporeactivity at the peak of the pollen season may be suggested. Regarding the clinically established long-lasting effects, the core mechanism is likely to consist of T regulatory (Treg) cell activation. In particular, Treg cells differentiate from naive T cells after application of soluble antigens to the mucosae, a crucial factor being the tolerogenic function of dendritic cells, and exert a suppressive effect on both Th1 and Th2 responses. Moreover, at least for the type 1 cells (Treg1), a production of IL-10 with consequent down-modulation of the immune response has been reported. Another characteristic of sublingual immunization is the absence of effectors cells, viz., mast cells, basophils, and eosinophils, in the oral mucosa of allergic subjects, which account for the excellent tolerability of SLIT.