C-reactive protein (CRP) is an acute-phase protein featuring a homopentameric structure and Ca-binding specificity for phosphocholine (PCh). Expression of CRP is regulated mainly at the transcriptional level with interleukin-6 being the principal inducer of the gene during the acute phase. The crystal structure of CRP has been determined and the topology and chemical composition of its ligand-binding site determined. The wide distribution of PCh in polysaccharides of pathogens and in cellular membranes allows CRP to recognize a range of pathogenic targets as well as membranes of damaged and necrotic host cells. CRP bound to a multivalent ligand can efficiently initiate the assembly of a C3 convertase through the classical pathway and thus decorate the surface of the ligand with opsonic complement fragments. However, the protein does not favor the formation of a C5 convertase and therefore, CRP-initiated complement activation does not mediate acute inflammatory reactions and membrane damage. CRP also interacts with Fc receptors on phagocytic cells and acts as an opsonin. Other CRP-initiated signals through interactions with neutrophil Fc receptors have an overall anti-inflammatory effect. Thus, the main biological function of CRP appears to be host defense against bacterial pathogens and clearance of apoptotic and necrotic cells. Protection from lethal bacterial infection, from complement-induced alveolitis, and from endotoxemia has been confirmed in vivo using transgenic mice. Additional functions, including participation in atherogenesis and pathogenesis of myocardial injury after myocardial infarction have been reported. However, the weight of the evidence is that CRP like other acute-phase proteins is a component first line of innate host defense.