C4A and COVID-19: In this study, the levels of total immunoglobulins (IgG, IgM, IgA), complement (C3, C4),lymphocyte subsets (CD3+ cell,CD4+ cell,CD8+ cell, NK cell, CD19+ cell and CD45+ cell) and cytokines (IL-2, IL-4, IL-5, IL-6, IL-8, IL-10, IL-17, IL-12p, IL-1β, TNF-α, IFN-α and IFN-γ) were retrospectively analyzed in COVID-19 patients.