Fig. 1.

Schematic presentation of possible mechanisms of probiotic immunomodulation effects in the intestine. Probiotics trigger immunomodulation through direct and indirect interaction with intestinal epithelial cells. Dendritic cells extend their dendrites between intestinal epithelial cells (IECs) and might directly sample and process probiotics in the gut lumen, leading to activation of innate and adaptive immune responses. Dendritic cells, present immediately below M cells, engulf probiotics, resulting in the maturation of DCs and may derive B cells into plasma cells. Additionally, after the interaction of probiotics with macrophages and dendritic cells presented in lamina propria, these cells are activated and induce NK cell activation, which leads to IFN-γ elevation to defend against viruses. Upon the interaction of probiotics' PAMPs with different types of toll-like receptors (TLRs), nuclear factor-κB (NF-κB)-mediated antiviral gene expression is stimulated. Eventually, active immune cells migrate to sites of infection through lymphatic and circulatory systems to defend against respiratory viruses.