TIMP1 and infection: Only two cytokines, tissue inhibitor of metalloproteinases 1 (TIMP-1) (P < 0.0001) and platelet-derived growth factor subunits BB (PDGF-BB) (P < 0.05) showed significantly elevated levels in children with multiple ARTIs compared with children with either a single ARTI or no infection (Fig. 5a, b, and Supplementary Fig. 4, nonparametric Kruskal-Wallis Test).