BTC and COVID-19: To the best of our knowledge, this is the first time that markers ADA, BTC, DPP6, EDIL3, ENTPD2, Flt3L, LIF, and LRP1 are reported to be upregulated in severe COVID-19 patients; hence, they can be proposed as potential biomarkers for severe cases of COVID-19; for example, LIF is not normally produced in the lungs, but when alveolar macrophages, while patrolling the blood–air barrier, encounter a virus, they release inflammatory cytokines as an alarm, which triggers LIF production.