PTGDR2 and allergic disease: In clinical trials, CRTH2 antagonists have been reported to be effective in the treatment of asthma and allergic diseases (18–20), especially in those with more eosinophils, but recent studies with higher level of evidence did not show a statistically significant improvement in the treatment of asthma (21–23), thus the clinical benefits of CRTH2 antagonists remained to be confirmed.