+C3d is a fragment born from the [Complement](/wiki/Complement) system, a vital sentinel in immunity. This small protein binds to [B Cell](/wiki/B_Cell) receptors, often acting as a bridge to amplify their response against invaders.
+## See also
+- [Immunity](/wiki/Immunity)
+- [C3](/wiki/C3)
+- [Antibody](/wiki/Antibody)
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