They are a superfamily of multifunctional conserved proteins, some of which are components of the humoral arm of innate immunity and behave as functional ancestors of antibodies (Abs).

[2] Pentraxins are characterised by calcium dependent ligand binding and a distinctive flattened β-jellyroll structure similar to that of the legume lectins.

[3] The name "pentraxin" is derived from the Greek word for five (πέντε, pente) and axle (axis) relating to the radial symmetry of five monomers forming a ring approximately 95Å across and 35Å deep observed in the first members of this family to be identified.

The protein resembles antibody and performs several functions associated with host defence: it promotes agglutination, bacterial capsular swelling and phagocytosis, and activates the classical complement pathway through its calcium-dependent binding to phosphocholine.

SAP binds to various lipoprotein ligands in a calcium-dependent manner, and it has been suggested that, in mammals, this may have important implications in atherosclerosis and amyloidosis.