Though IL-9 deficiency seemed to had no significant effect on the final clearance of Pneumocystis organisms, IL-9−/− mice demonstrated a significantly decreased Pneumocystis burden compared to WT mice [(1.39 ± 0.10) × 108 vs. (7.91 ± 1.26) × 107 copies/lung, P < 0.01] (Figure 1B) at 3-week postinfection, suggesting that IL-9 deficiency might contribute to fighting against Pneumocystis infection. Here, IL9 is linked to Pneumocystis infectious disease.