Figure 1.

Architecture of the mucosal surface. The mucosal surface of the intestine contains a single layer of epithelial cells. The monolayer of epithelial cells is fortified by a layer of mucus (yellow) produced by Goblet cells (blue cells with yellow granules). This thick mucus layer contains membrane bound and secreted mucins. The antimicrobial peptides (red) secreted by Paneth cells (blue cells with red granules) reside in the thick mucus layer, providing another form of protection against both pathogenic and commensal bacteria. Antimicrobial peptides include defensins, cathelicidins, and histatins. Plasma B cells (light green) reside in the subepithelial region and produce secretory IgA (blue and red antibody). Secreted IgA is found in the subepithelial region and the lumen. Resident microbiota (green) reside in the outer mucus layer, providing yet another barrier to pathogenic infection. The majority of resident microbiota belong to two phyla – Firmicutes and Cytophaga–Flavobacterium – Bacteroidetes. The seal between epithelial cells is maintained by tight junctions (orange bars). Tight junctions are dynamic structures composed of zonula occludens and junctional adhesion molecules.