Fig. 5.

Mucus produced by goblet cells forms a continuous layer in relatively close proximity to the epithelial cell (Epi) surface of the colon. The mucus layer is discontinuous in the small intestine (not shown). In the colon, there is an inner dense layer as well as a less dense region closer to the lumen [40, 41]. The mucus layer provides a physical barrier to limit contact between the lumenal microbes and host cells. In addition, antimicrobial peptides (AMPs) and secretory immunoglobulin A (IgA) are present at the epithelial surface and embedded in mucus. These immunologically active molecules contribute to the barrier function of mucosal surfaces