IL21 and interstitial lung disease: We hypothesize that in PSO/PSA patients, abnormal activation of CD4 + Th17 cells and excessive production of cytokines like IL-17 cause skin and joint inflammation, while also inducing lung inflammation and fibrosis through the proliferation of fibroblasts and production of cytokines such as TNF-α, IL-6, IL-21, IL-22, and IL-23, leading to ILD [35, 36].