CD4 and viral infectious disease: Considering that several studies reported that oral administration of probiotic strains increased protection against influenza virus infection in mice by increasing NK cell activity and IFN-γ production in lung [17,21,25], we can speculate that the mobilization of CD3+CD4+IFN-γ+ T cells from the intestine to the airways and the improved production of IFN-γ could be involved in the protective effect against viral infections induced by Lr1505 that was observed in clinical studies [7].