The under-researched barrier function of the mucus layer

Abstract

The mucus layer covers all the internal surfaces of the body. The surfaces communicate with the external environment. The functions of the mucus layer are determined by its components, including glycoproteins that provide physical and chemical protection to the epithelium. The glycoproteins also perform the exchange function with the external environment. Even at the dawn of evolution, glycoproteins were exteriorly organized to protect the first multicellular animals from external microorganisms, pathogens, and toxins. It is interesting to note that the structure of the glycoproteins has similarities with the structure of antibodies, especially in terms of the relatively constant polypeptide chain and its glycosylated sections, containing oligosaccharide chains with different variants of monosaccharides. The review discusses modern concepts of the mucus layer evolution, the structure of glycoproteins, the peculiarities of its synthesis, degradation, and under-researched functions of the mucus layer. It is assumed that the mucus layer has virucidal and bactericidal capabilities due to circulating enzymes of the digestive tract, which can accumulate in the mucus layer and degrade any microorganisms, regardless of their variation, mutations, and recombination. Therefore, the normal production of digestive tract enzymes can provide non-specific protection from external pathogens entering through open systems of the body. Understanding these processes can significantly limit the spread of existing and new infections.