Biological barrier: composed of the resistance of normal bacteria to the colonization of pathogenic bacteria and the aggregation of bacteria
Chemical barrier: composed of mucus secreted by intestinal mucosal epithelial cells, digestive juices and bacteriostatic substances produced by intestinal parasitic bacteria, which can inhibit and kill pathogenic microorganisms
Physical (mechanical) barrier: composed of the remaining water layer on the mucosal surface, hydrophobic layer, adhesive mucus gel layer, intact intestinal epithelial cells and tight junctions between adjacent intestinal epithelial cells
Immune barrier:  composed of antibodies secreted by intestinal mucosal lymphoid tissues and intestinal plasma cells, and prevents pathogenic antigens from harming the body through cellular and humoral immunity
Intestinal barrier damage:
Biological barrier damage: the balance of bacteria is destroyed, and harmful bacteria multiply in large numbers, and produce toxins to destroy the mucosal system and even invade the body and even migrate the intestinal bacteria
Chemical barrier damage: reduced secretion of digestive enzymes from digestive fluid and reduced bactericidal ability of intestinal fluid 
Physical barrier damage: intestinal villi sparse atrophy and shedding; The intestinal mucosa becomes atrophic and thin, and the new repair ability of the mucosa becomes weak
Immune barrier damage: impaired immune cell differentiation, impaired immune tissue, reduced mucosal immune capacity impaired intestinal barrier leads to the growth potential of pigs is not fully exploited, intestinal health problems caused by piglet mortality up to 15-20