Figure 2.

Epithelial defense in the nose first consists of mechanical barriers including mucociliary flow and tight junctions between respiratory epithelial cells that limit stimulation of pattern recognition receptors (PRRs) through diminished exposure and access. PRRs recognize pathogen-associated molecular patterns (PAMPs), which are conserved molecular patterns common among pathogens; recognition of PAMPs by PRRs serves as a “danger” signal to the host immune system. PRRs also identify cellular “damage” through detection of debris from necrotic cells and the combined recognition of danger and damage signals sets in motion a response consisting of endogenous antimicrobial, antiviral, and antiproteinase products designed to aid pathogen clearance. PRR activation also triggers the release of chemokines and cytokines mediating the inflammatory response that attracts innate cellular defenses such as neutrophils. If sufficiently strong, PRR stimulation also sets in motion and ultimately determines the nature of the acquired immune response. Although not considered host defense molecules, PAR receptors (not depicted) are also present on many of the cell types present in the nasal mucosa. (Illustration by William E. Walsh, MD, CMI, 2008 William Walsh; used with permission.)