{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,13]],"date-time":"2026-04-13T05:51:07Z","timestamp":1776059467460,"version":"3.50.1"},"reference-count":46,"publisher":"Wiley","issue":"1","license":[{"start":{"date-parts":[[2026,4,12]],"date-time":"2026-04-12T00:00:00Z","timestamp":1775952000000},"content-version":"vor","delay-in-days":101,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"},{"start":{"date-parts":[[2026,1,1]],"date-time":"2026-01-01T00:00:00Z","timestamp":1767225600000},"content-version":"tdm","delay-in-days":0,"URL":"http:\/\/doi.wiley.com\/10.1002\/tdm_license_1.1"}],"funder":[{"DOI":"10.13039\/501100001773","name":"University of New South Wales","doi-asserted-by":"publisher","id":[{"id":"10.13039\/501100001773","id-type":"DOI","asserted-by":"publisher"}]}],"content-domain":{"domain":["onlinelibrary.wiley.com"],"crossmark-restriction":true},"short-container-title":["Journal of Immunology Research"],"published-print":{"date-parts":[[2026,1]]},"abstract":"<jats:p>Following severe acute respiratory syndrome coronavirus 2 (SARS\u2010CoV\u20102) infection, the complement pathway becomes activated, generating the stable split product C3d. Although C3d can modulate B cell responses through complement receptor 2 (CR2) in experimental systems, it remains unclear how circulating C3d relates to antibody responses after SARS\u2010CoV\u20102 infection in humans. Here, we examined plasma C3d levels and their associations with SARS\u2010CoV\u20102\u2013specific antibody responses and clinical characteristics in convalescent adults and children. In adults, older age, male sex, and a history of severe COVID\u201019 were independently associated with higher C3d levels. C3d concentrations positively correlated with receptor\u2010binding domain (RBD)\u2010specific antibody\u2010dependent cellular phagocytosis (ADCP), neutralizing antibody (nAb) titer, and anti\u2010spike IgM, IgG, IgG1, and IgG3 levels. In children, C3d levels were significantly lower than in adults and correlated with anti\u2010spike and anti\u2010RBD IgG titers. These findings demonstrate that systemic complement activation, reflected by circulating C3d, is positively associated with SARS\u2010CoV\u20102 antibody responses in both age groups. As this cross\u2010sectional study cannot determine causality, further mechanistic and longitudinal studies are needed to define the role, if any, of C3d in shaping adaptive immunity to SARS\u2010CoV\u20102.<\/jats:p>","DOI":"10.1155\/jimr\/7429749","type":"journal-article","created":{"date-parts":[[2026,4,13]],"date-time":"2026-04-13T05:09:46Z","timestamp":1776056986000},"update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["C3d Complement Activation and Antibody Immunity in Convalescent COVID\u201019: Insights From Adults and 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