Displays several functions associated with host defense: it promotes agglutination, bacterial capsular swelling, phagocytosis, and complement fixation through its calcium-dependent binding to phosphorylcholine. Binds 2 calcium ions per subunit. Homopentamer. Pentraxin (or pentaxin) have a discoid arrangement of 5 non-covalently bound subunits. Exists as a dimer under reducing conditions. Glycosylated. Shows sequence similarity to serum amyloid P-component-type pentraxins but binds to phosphatidylcholine. Belongs to the pentraxin family.