Mucosal immunology

Mucosal immunology is the study of immune system responses that occur at mucosal membranes of the intestines, the urogenital tract, and the respiratory system.^[1] The mucous membranes are in constant contact with microorganisms, food, and inhaled antigens.^[2] In healthy states, the mucosal immune system protects the organism against infectious pathogens and maintains a tolerance towards non-harmful commensal microbes and benign environmental substances.^[1] Disruption of this balance between tolerance and deprivation of pathogens can lead to pathological conditions such as food allergies, irritable bowel syndrome, susceptibility to infections, and more.^[2]

The mucosal immune system consists of a cellular component, humoral immunity, and defense mechanisms that prevent the invasion of microorganisms and harmful foreign substances into the body. These defense mechanisms can be divided into physical barriers (epithelial lining, mucus, cilia function, intestinal peristalsis, etc.) and chemical factors (pH, antimicrobial peptides, etc.).^[3]

Function

The mucosal immune system provides three main functions:

• First line of defense from harmful antigenic structures and infection.^[3]
• Prevents systemic
  commensal bacteria and food antigens.^[2]
• Regulates appropriate immune responses to pathogens.^[2]

Physical barrier

Mucosal barrier integrity physically stops pathogens from entering the body.^[4] Barrier function is determined by factors such as age, genetics, types of mucins present on the mucosa, interactions between immune cells, nerves and neuropeptides, and co-infection. Barrier integrity depends on the immunosuppressive mechanisms implemented on the mucosa.^[3] The mucosal barrier is formed due to the tight junctions between the epithelial cells of the mucosa and the presence of the mucus on the cell surface.^[4] The mucins that form mucus offer protection from components on the mucosa by static shielding and limit the immunogenicity of intestinal antigens by inducing an anti-inflammatory state in dendritic cells (DC).^[5]

Active immunity

Because the mucosa surfaces are in constant contact with external antigens and microbiota many immune cells are required. For example, approximately 3/4 of all lymphocytes are found in the mucous membranes.^[3] These immune cells reside in secondary lymphoid tissue, largely distributed through the mucosal surfaces.^[3]

The

oral tolerance.^[11]

If mucosal barrier homeostasis has not been violated and invasive pathogens are not present, dendritic cells induce tolerance in the gut due to induction of Tregs by secretion of TGF-β and retinoic acid.^[17] These Tregs further travel to the lamina propria of villi through lymphatic vessels. There, Tregs produce IL-10 and IL-35, which affects other immune cells in the lamina propria toward a tolerogenic state.^[17]

However, damging the

Because of its front-line status within the

• Microfold cell

References