Fig. 1.

Proposed mechanism of lymphopenia in patients with COVID-19. A host specific, exuberant PGD₂ response early in infection, initiates DP₁ signaling, which inhibits the dendritic cell function by downregulating CCR7, leading to a weak T cell response. PGD₂/DP₂ signaling stimulates respiratory ILC2 and Th2 cells, which secrete IL-13. IL-13 stimulates proliferation of MDSC cells, thereby downregulating the pathogen specific T cell responses. Excessive PGD₂ action via DP₁ receptors during the incubation period and DP₂ receptors during the symptomatic stage leads to lymphopenia. Lymphopenia is a predictor of morbidity and mortality in COVID-19.