Immunological Features of Post-COVID Syndrome Associated with Cytomegalovirus Infection: The Role of Neutrophils and the Complement System

Post-COVID syndrome, affecting up to 48 % of patients after COVID-19, is characterized by persistent immune dysregulation, including innate immunity dysfunction and increased susceptibility to reactivation of chronic infections. Cytomegalovirus (CMV), which persists in 60—90 % of the population, can cause severe complications under immunosuppressive conditions, such as hepatitis, encephalitis, and multiorgan failure. The relevance of this study lies in investigating the relationship between CMV reactivation and the immunopathogenesis of post-COVID syndrome, particularly in the context of neutrophil dysfunction and complement system impairment. The aim of this study was to evaluate the features of neutrophil phagocytic activity and complement system activity in patients with post-COVID syndrome and concurrent CMV infection. Methods included assessment of neutrophil phagocytic function using 1.7 μm latex particles: counting phagocytic cells per 100 neutrophils, calculating the phagocytic index, phagocytic number, and phagocytic intensity via light microscopy. Neutrophil bactericidal activity was analyzed using the standard nitroblue tetrazolium (NBT) test. Complement system component levels were measured by enzyme-linked immunosorbent assay (ELISA). The findings will help identify specific impairments in phagocy­tic function and complement-mediated immunity associated with CMV reactivation in post-COVID syndrome. The results of this study may provide a basis for developing personalized immunomodulatory approaches for this patient population.