Proposed mechanism for the improvement of antiviral immunity by orally administered lactobacilli. Enhancement of intestinal antiviral immunity by Lactobacillus rhamnosus CRL1506 (A). Enhancement of intestinal and respiratory antiviral immunity by Lactobacillus rhamnosus CRL1505 (B). Orally administered L. rhamnosus CRL1505 stimulates the Th1 response in the gut and to induce mobilization of Th1 cells from inductive sites in the gut to effector sites in the respiratory tract. These activated Th1 cells would produce cytokines (IFN-γ) able to stimulate the activity of local respiratory immune cells such as alveolar macrophages, NK cells and CD11c+CD11blowCD103+ dendritic cells. These previously activated immune cells would be able to efficiently phagocyte pathogens that reach the alveolar space, induce specific immune responses and increase the resistance to bacterial and viral respiratory infections.