Structure and Function of Mucosal Surfaces

The mucosal surfaces of the body are the areas where important absorptive and excretive functions occur. The innate defense system consists of three components: mechanical, chemical, and cellular. The first defense that an invading pathogen would encounter is the preepithelial barrier. Mucins form two lines of preepithelial defense, such that the secreted gel overlies the mucins forming part of the glycocalyx on the apical surface of the epithelial cells. The most important chemokine released from the epithelial cells is interleukin-8 (IL-8), which, as well as being an effective chemoattractant for granulocytes, stimulates mucin secretion by goblet cells. Some microorganisms can utilize the mucus layer for protection. Several proteinase inhibitors are produced by epithelial cells and, where present, submucosal glands, e.g., in the airways. These inhibitors form an important part of the preepithelial defenses and the innate immune system. Secretory leukocyte proteinase inhibitor (SLPI) is one such epithelial secretion, along with elafin (SKALP), which is an elastase inhibitor with 42% sequence homology to SLPI. A key role for the preepithelial barrier is to prevent microbial adherence by interfering with microbial adhesins and toxins. Several enzymes are secreted by the epithelial cells into the external secretions. A key role for the preepithelial barrier is to prevent microbial adherence by interfering with microbial adhesins and toxins. As well as cell membrane-associated mucin, the apical surface of the epithelial cells has other molecules presented to approaching microbes.