Hypothetical timeline and interactions of events in the formation of nasal polyps. (Left) The damaged epithelium responds to injury from pathogens, proteases, and irritants. Abbreviations: BAFF, B cell–activating factor; CCL23, chemokine (C-C motif) ligand 23; DC, dendritic cell; IL, interleukin; ILC2s, type 2 innate lymphoid cells; MCP-4, chemokine (C-C motif) ligand 13, also known as CCL13; PAMP, pathogen-associated molecular patterns; Th2, T helper cell type 2; tPA, tissue plasminogen activator; TSLP, thymic stromal lymphopoietin; VCAM1, vascular cell adhesion molecule 1.