Interestingly, the percentage of Th17 cells in CD4+ lymphocytes was significantly higher in the IL-9−/− mice than WT mice in the BALF [(5.14 ± 0.83 vs. 26.42 ± 1.71)%, P < 0.001] and lungs [(3.93 ± 0.31 vs. 7.31 ± 0.64)%, P = 0.003] at 3-week postinfection, when Pneumocystis burden peaked during infection. Here, IL9 is linked to infection.