Serum amyloid P component (SAP) is the identical serum form of amyloid P component (AP), a highly preserved plasma protein named for its ubiquitous presence in amyloid deposits. As a normal plasma protein first identified as the pentagonal constituent of in vivo pathological deposits called "amyloid". Serum amyloid P component represents another member of the pentraxin family, a highly conserved group of molecules that may play a role in innate immunity. SAP is a key negative regulator for innate immune responses to DNA and may be partly responsible for the insufficient immune responses after DNA vaccinations in humans. SAP suppression may be a novel strategy for improving efficacy of human DNA vaccines and requires further clinical investigations.