Functional imbalance in Th1/Th2 cell response toward allergens is a recognized hallmark of allergic patients and a major role of dendritic cells (DCs) in redirecting T-cell phenotypes after specific immunotherapy has been suggested. This study investigates the proliferative and cytokine responses of T cells cocultured with monocyte-derived DCs (MoDCs) after allergen stimulation in birch-allergic patients compared with controls and investigates whether sublingual immunotherapy (SLIT) could change the DC-driven immune response. T cells were stimulated with the major birch pollen allergen (nBet v1) and MoDCs from eight birch-allergic patients with seasonal allergic rhinitis and eight nonallergic controls. Proliferation and cytokine production were measured before and after one course of SLIT with birch allergoid. Significantly lower levels of proinflammatory (IL-1beta, p = 0.027; IL-6, p = 0.030; TNF-alpha, p = 0.019) and Th1 (interferon gamma, p = 0.032; IL-12, p = 0.05) cytokines were measured in supernatants of T cells and MoDCs cultures from allergic patients compared with nonallergic controls. After SLIT, significant increase in IL-12 (p = 0.039), IL-1beta (p = 0.040), IL-6 (p = 0.041), TNF-α (p = 0.048), and IL-10 (p = 0.048) and significant decrease in IL-13 (p = 0.001) were observed. MoDCs/T-cell cocultures, pulsed with the specific allergen, produced lower quantities of proinflammatory and Th1 cytokines in allergic patients compared with healthy subjects, suggesting an allergen-specific impairment of natural immunity and Th1 immune response. A single course of SLIT was able to enhance allergen-specific innate immunity and to modify lymphocyte response, promoting Th1 and T-cell regulatory activity.