Question

What are the physical barriers which act to maintain the epithelial barrier to microorganisms in the intestine? If that barrier breaks down, how does this change the way mucosal immune defenses act? What are defensins? What cells produce and secrete them? What is secretory IgA and where does it come from?

Answer 1

- The physical barriers which acts to maintain the epithelial barrier to microorganisms in the intestine consists of mucus, intestinal epithelial cells and microbiota of the intestine.

The mucus is a gel like slippery secretion produced by the goblet cells of the intestinal epithelial cells which line the entire mucosa of the intestine. The mucus prevents the microorganisms from coming in direct contact with the mucosal surface of the intestine but allowing the smooth passage of intestinal luminal contents of digested food material.

The intestinal epithelial cells have tight junctions which do not allow the entry of microorganisms or their toxins into the epithelial cells of the intestine from the luminal side while allowing the passage of nutrients and ions into the epithelial cells from the lumen of intestine.

The microbiota which consists of commensal microorganisms prevents the growth of pathogenic microorganisms into the intestine as well as helps in the proliferation of the intestinal epithelial cells thus helping in maintaining the intestinal epithelial barrier.

- If the intestinal epithelial barrier breaks, the pathogenic microorganisms and their toxins gain entry into the epithelial cells and capillaries from the lumen of the intestine. This can cause the activation of immune system due to the toxins which can result in conditions like autoimmue diseases like celiac disease, inflammatory bowel disease. The disruption of intestinal epithelial barrier can also cause microbial infection as well sepsis due to the toxins released by the pathogenic microorganisms.

- Defensins are antimicrobial proteins rich in amino acid cysteine. Defensins are produced by specialized intestinal cells called paneth cells which are located at the base of intestinal crypts. Defensins can destroy bacteria, viruses and fungi by directly increasing their membrane permeability as well as by activating other immune cells like macrophages which helps in the destruction of these pathogenic microorganisms.

- Immunoglobulin A (IgA) are are also called as secretory antibodies as they are found in the mucus secretions of the body like saliva, tears, gastrointestinal secretions, colostrum. It plays an important role in the mucosal immunity thus an important factor in intestinal epithelial barrier as well preventing the invasion by pathogenic microorganisms or their toxins in the lumen of the gastrointestinal tract.