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. Heteropentamer. Discoid arrangement of 5 non-covalently bound subunits 1, 2, 3 and 4. Glycosylated. Belongs to the pentraxin family.