+**C4d** is a stable fragment of the [Complement](/wiki/Complement) protein C4, often found deposited in tissues following classical complement pathway activation. Its presence serves as a crucial [Biomarker](/wiki/Biomarker) for an active antibody-mediated immune response, particularly significant in the diagnosis of organ transplant rejection.
+## See also
+- [Immunity](/wiki/Immunity)
+- [Inflammation](/wiki/Inflammation)
+- [Antibody](/wiki/Antibody)
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