ENTPD1 and viral infectious disease: Figure 2 demonstrates the increased proportion of CD39+ expressing cells in lymphocytes, identified by its scattering properties and CD45 positivity, in severe courses of COVID-19. CD39 overexpression was particularly observed in patients with severe COVID-19. In conclusion, the presented results in Table 3 and Figure 2 indicate that the CD39 signaling pathway may be upregulated, and lymphocyte function may be impaired in the early phase of acute viral infection, leading to T-cell exhaustion and increased complications of COVID-19.