Figure 1.

Major components of the mucosal barrier. The luminal stream is separated from the epithelium by the mucus layer, which is the physical substrate for a concentration gradient of antimicrobial molecules which acts as a repellent, protecting the stem cell compartment from microbial damage. The epithelium itself is the boundary between host and luminal environment, replaced entirely every 3–5 days and therefore dependent on proliferative drivers and repair factors. Organisms or their component molecules which traverse the epithelial boundary are then the target of innate and anamnestic immune responses.