Abstract

Purpose Humoral immunity proteins—immunoglobulins, complement proteins, and antimicrobial peptides—have key antimicrobial and immunomodulatory functions in sepsis. We hypothesised that their circulating levels are lower in non-survivors, potentially resulting in impaired bacterial clearance and persistent or recurrent infections. Methods We performed a systematic review and meta-analysis evaluating differences in humoral immunity proteins between survivors and non-survivors in adult patients with sepsis. PubMed and Embase were searched without date restrictions. Random-effects meta-analyses were used to estimate pooled standardised mean differences (SMD) with 95% confidence intervals (CI). Sensitivity analyses included data from the MIMIC-IV ICU database, and further supplemented by three proteomic studies. Results Thirty-six studies including 6,330 patients were analysed. Thirteen reported on immunoglobulins, 17 on complement proteins, and 7 on the antimicrobial peptide heparin-binding protein (HBP). Survivors had significantly higher levels of complement proteins C3 (SMD 0.53 [0.07–0.99]) and C4 (SMD 0.51 [0.09–0.94]) compared to non-survivors. Conversely, C4a (SMD − 1.17 [–1.77 to − 0.56]) and IgA (SMD − 0.21 [–0.39 to − 0.03]) were significantly lower in survivors. No differences were found for IgG (SMD 0.00 [–0.18 to 0.18]), IgM (SMD − 0.02 [–0.13 to 0.08]), C5, C5a, or HBP. Sensitivity analyses using MIMIC-IV (n = 2,452) and proteomic datasets supported these findings. Proteomic data revealed early depletion of classical complement components (C3, C4B) and regulatory proteins in non-survivors. Conclusion Sepsis non-survivors exhibit lower C3 and C4 levels and higher C4a, consistent with complement activation and/or depletion. Complement proteins may serve as potential biomarkers and therapeutic targets in sepsis.