Figure 2.

Structure of the luminal epithelial barrier in healthy and asthmatic airways. In the healthy state, microbes entrapped in mucus are attacked by antimicrobial agents (exemplified by a pink symbol). The viscous mucus gel is constantly moved onwards by the coordinated beating of the cilia. The cilia are covered by a dense meshwork of glycoproteins—the glycocalyx. Only particulate matter (e.g., viruses) (red), which are smaller in diameter than the glycocalyx pore size, can reach target receptors (blue) on the apical membrane; larger particles/viruses can only bind to receptors on cilial tips, respectively, in the outer part of the glycocalyx. In the asthmatic state, the composition of mucus is altered. Overproduced mucin MUC5AC inhibits antimicrobial activities, and higher mucin content leads to mucus thickening, dehydration, and shrinking of the periciliary layer. As a consequence, cilia are bent, and their beating becomes disordered. Movement of the thickened mucus plugs is impeded, resulting in obstruction of airways.