Complement C3d (C3d)

Complement C3d is a 35 kDa proteolytic fragment of Complement C3, representing the final degradation product of C3b. It is covalently attached to cell surfaces and tissues at sites of complement activation, serving as a long-lived marker of local inflammation [1, 8]. C3d plays a critical role in bridging innate and adaptive immunity by binding to Complement Receptor 2 (CR2/CD21) on B cells and follicular dendritic cells, which significantly lowers the threshold for B-cell activation and enhances antigen presentation [10, 12]. In various autoimmune and inflammatory diseases, such as systemic lupus erythematosus and C3 glomerulopathy, excessive C3d deposition contributes to tissue damage [1, 8]. Therapeutically, C3d is targeted by bifunctional fusion proteins, such as ADX-097, which use anti-C3d antibodies to deliver complement regulators specifically to diseased tissues, providing localized inhibition while sparing systemic complement function [1, 8]. Additionally, C3d is widely utilized as a molecular adjuvant in vaccine development to boost humoral immune responses by mimicking natural opsonization [10, 16].

Other names

C3d fragmentComplement component 3dComplement C3d fragmentalpha-chain fragment of C3

Localized complement inhibition via tissue-bound C3d targeting to deliver complement regulators (e.g., Factor H) to sites of injury, or enhancement of B-cell activation as a molecular adjuvant.