ACE2 and Respiratory tract infection: Besides, SARS-CoV-2 mainly infect human angiotensin-converting enzyme 2 (ACE2), which is mainly expressed in tongue epithelial cells, while the viral load of SARS-CoV-2 in posterior oropharyngeal saliva samples was highest during the first week of symptom onset (Lirong et al., 2020), which further reveals that homeostasis of oropharyngeal mucosa that has an impact on the programming of the innate immune system could play an important role as a frontline defense and protect human host from respiratory tract infections including SARS-CoV-2.