Fig. 2
Acquired microbial immunity. The human immune consists of innate and acquired immunity, which is mainly carried out by T and B cells. The main strategies of adaptive immunity are active and passive immunization. In active immunity, natural immunity can be acquired by direct infection with the pathogen, while vaccination with the antigen is the artificial way. Passive immunization is mainly achieved by natural means, such as breastfeeding, or artificial means, such as immunoglobulin injections. Commensal microbiota described here can provide another form of acquired defence and regulating power against pathogens (commensal microbiota immunity). Correspondingly, maternal human milk oligosaccharides (HMOs), acquired through maternal reproductive transmission and exposure, can enhance the colonization of beneficial microbes under natural conditions. Under artificial conditions, fecal microbiota transplantation (FMT),660,661,662,663,664 probiotics,665,666,667,668,669 prebiotics,670 synbiotics671,672 and postbiotics673,674,675,676 can be used to acquire this immunity. Commensal microbiota immunity strengthens cellular barriers and regulates immune cells through metabolites such as short-chain fatty acids. They train and educate the immune system as a competitor while providing colonization resistance against foreign and established pathogenic microbes. The decline of commensal microbiota immunity increases the risk of skin and food allergies,677 asthma,548 type 1 diabetes (T1D),678 pathogenic overgrowth (such as Clostridium difficile),667,679,680,681,682,683,684,685,686,687 and susceptibility to inflammatory bowel disease (IBD)555,688,689,690,691,692,693,694,695 and other potential diseases696,697
