REN and pulmonary fibrosis: Besides, the used keywords were stem cell, amnion, amniotic epithelial cell, exosome, regenerative medicine, cell therapy, immunomodulation, inflammation, lung fibrosis, renin–angiotensin–aldosterone system, alveolar fluid, coagulation, coagulopathy, COVID-19, SARS-CoV-2, drug delivery, gene modification and immune system focusing on the biology of hAECs and the possible therapeutic role of hAECs in COVID-19.