The elevation of acute phase reactants (APRs) is a nonspecific host response to infection, inflammation, and tissue injury. The major biologic function of APRs is to restore homeostasis and to improve survival. Measuring the alterations in APRs can be a useful clinical marker when an infection or inflammatory response is suspected. Serum levels of reactants like fibrinogen and complement proteins increase as part of the inflammatory response, but the increase is trivial and does not contribute to the differential diagnosis or the evaluation of therapeutic responsiveness. By contrast, C-reactive protein (CRP) concentrations increase markedly with acute invasive infections which parallel the severity of inflammation or tissue injury. This advantage makes CRP a useful marker for the presence of disease, response to therapy, and ultimate recovery.