Probiotics are live microorganisms which when administered in adequate amounts confer a health benefit on the host. Through interaction with the host gastrointestinal tract, they are able to influence its innate and adaptive immune response. Probiotics affect the ability of phagocytosis of macrophages and other cells, affect the number and efficiency of NK cells, stimulate the production of cytokines and IgA antibodies, affect the Treg cells via DC cells, and balance Th1 and Th2 sub-populations. Probiotics are strain specific. L. reuteri, for example, inhibited the production of cytokine IL-8, while the strain L. plantarum 299v stimulated it. Some probiotics have the ability to control the immune response, depending on the health status of the host. L. rhamnosus GG stimulated phagocytosis in healthy individuals and inhibited it in milk-hypersensitive patients. The most commonly used probiotics are G + lactic acid bacteria from the genus Lactobacillus and Bifidobacterium, but also G-bacteria, for example E. coli Nissle 1917, are very promising, as they show greater immunostimulatory effects; probably due to different PAMPs on their surface. Probiotics, with their immunomodulatory properties, show great potential for use in the treatment of various diseases (diarrhea, ulcerative colitis, hypersensitivity, allergies, atopic dermatitis, autoimmune diseases, etc.) and they would also be useful as adjuvants to oral vaccines.