C-reactive protein is a protein produced in the liver in response to inflammation. It binds to dead and dying cells (both those of the individual and any foreign cells) and some types of bacteria to activate the complement system. This marks those cells for destruction by white blood cells.
Originally discovered in 1930, it was first thought to be a foreign protein itself as it was almost always only found in quantity in the plasma in a patient with an infection or cancer - never in healthy patients. However, the liver is capable of producing huge quantities of the protein in response to inflammation - a 50,000 fold increase within two hours of onset.
Viruses do not give rise to a similar response as it appears other moderators in the immune system can suppress C-reactive protein in such cases.
C-reactive protein can be tested for, with very high levels indicating a bacterial infection. It is far more reliable than sed rate in detecting infection. However, levels on the high side of normal can be an indicator of risk for heart disease, diabetes mellitus, colon cancer and high blood pressure. Very low levels can indicate liver failure.
C-reactive protein levels are also useful in diagnosing autoimmune diseases as C-reactive protein levels generally remain low while other indications of inflammation are high.