Which probiotic is best for the immune system? The gastrointestinal tract or GT is the most microbiologically active ecosystem that contains a mass of bacteria that is crucial for the maturation of immune cells in humans. A large number of bacteria from the microbiota and others that reach the gut through food intake coexist with each other. The intestinal microbiota also doesn’t interact directly with the epithelial cells; the microbiota then stimulates the maturation as well as the functionality of the immune cells through their metabolites. There exists a group of beneficial bacteria known as probiotics. They were defined as “Live microbial feed supplements that affect the host, improving the intestinal microbial balance�. Many kinds of probiotic bacteria are members of the intestinal microbiota incorporated into foods to improve gut health by maintaining the gastrointestinal microbial balance. Some of the most common types of microorganisms used as probiotics are lactic acid bacteria (LAB), or Lactobacilli, Streptococci, Pediococcus, Enterococcus, Bifidobacteria, and some yeast like Saccharomyces boulardii. However, all bacteria cannot be probiotic, as they are strainspecific. The beneficial effects of probiotics have been used to improve the host health and treat different infectious and non-infectious pathologies in animals. Some of them are protection against infections, relief from irritable bowel symptoms, decreased gut inflammatory response, and prevention from allergies. There were also improvements in many other health conditions like diabetes, multi-drug resistant pathogens, irritable bowel symptoms, and many others. Gut Bacteria and Immunity