Fig. 4.

SARS-CoV-2 infection and host immune response in COVID-19 patients. In COVID-19 patients, the SARS-CoV-2 may infect ciliated cells, club cells, and basal cells expressing ACE2 and TMPRSS2 in lung epithelium and actively replicate in host cells. This could lead to activation of pro-inflammatory signaling and production of pro-inflammatory cytokines which subsequently attract both innate and adaptive immune cells including neutrophils, macrophages and T cells to the infection site to fight virus and virus-infected cells. Besides, the immune cells also release cytokines to attract more immune cells, creating a positive feedback loop of cytokine creation. Massive accumulation of pro-inflammatory cytokines producing-immune cells in the lungs could increase the severity of COVID-19 patients.