NCIt definition : C4d (380 aa, 41 kDa) is a fragment of complement component-4. This protein plays
a role in the regulation of the classical and lectin complement pathways. It may be
a biomarker for complement activation during antibody-mediated rejection of transplants
and in other pathological conditions.;
NCIt note : Human complement component C4 is polymorphic at two loci, C4A and C4B, thus C4d is
encoded by both of these genes. Both forms of complement component-4 can be cleaved
to generate C4 alpha chain and C4 beta chain. The alpha chain can be further processed
to release C4a anaphylatoxin and C4b. C4b is membrane bound and recruits C2a to form
the C4b2a form of the C3-convertase in the classical complement pathway. To deactivate
the C3-convertase, C4b is cleaved further to form the inactive fragments C4c and C4d.
C4d remains bound to the surface of the endothelium.;