Chapter 1. Lactic Acid Bacteria in Reducing Infection Via the Intestinal Immune System Chapter 2. Immunoglobulin A Drug Development as a Gut Microbial Regulator Chapter 3. Altered Humoral Immunity and Antigenic Evolution in Coronavirus Disease 2019 Chapter 4. Dynamics of Probiotic Strains in Human Small Intestinal Tract Chapter 5. Battling Helicobacter pylori with Our Microbial Weapons: The Emerging Era of Novel Microbiome-based Probiotics Chapter 6. Effect of Lacticaseibacillus paracasei Strain Shirota Intake on the Stability of Gut Microbiota in the Elderly Chapter 7. Effects of Probiotics on the Gut-Brain Interaction Chapter 8. Psychobiotics: Workable Approach in Sports? Chapter 9. Synbiotics and Gut Health: Science to Applications