Human CRP is a kind of nonimmunoglobulin serum substance, a heat labile b-globulin. It is classified in a superfamily of proteins termed pentaxins or pentraxins: cyclic, non-glycosylated structures composed of five apparently identical globular non-covalently linked subunits aggregated symmetrically. Each subunit is 23.05 kD (206 amino acids), with a total molecular weight of 117.5 kDa, and consists of 14 anti-parallel b-strands arranged in two b-sheets. CRP is an acute phase protein, originally identified and named for its ability to precipitate the C-polysaccharide of pneumococcus in the presence of calcium. It is the prototypic acute phase reactant whose presence in plasma or serum serves as a useful laboratory indicator of systemic inflammatory disease. Normally, CRP in human biological fluids is present in trace amounts (0.07-8.00 mg/L, median 0.6 mg/L). Stimulated by certain cytokines (IL-1a, IL-1b, TNF-a and b, and indirectly by IL-6), its synthesis by hepatocytes enhanced dramatically. During the acute phase response, CRP concentration can increase up to 1000-fold within a few hours. Among acute phase proteins, CRP is a fast-reacting, sensitive and the most easily measured one. It has a rapid response time, short half-life and large incremental change and its catabolism is not affected by the type of inflammation. Following acute tissue damage or during the course of infectious and non-infectious conditions, hepatic synthesis of CRP dramatically increases. Typically, mild elevations of CRP are seen in a variety of inflammatory conditions. Serum amyloid A (SAA) is another major acute phase protein whose response is highly correlated with that of CRP. Both CRP and SAA respond sensitively to several stimuli, but they differ in certain responses.