The acute phase response and C-reactive protein

The acute phase response—trauma, tissue necrosis, infection, inflammation, and malignant neoplasia induce a complex series of nonspecific systemic, physiological, and metabolic responses including fever, leucocytosis, catabolism of muscle proteins, greatly increased de novo synthesis and secretion of a number of ‘acute phase’ plasma proteins, and decreased synthesis of albumin, transthyretin, and high- and low-density lipoproteins. The altered plasma protein concentration profile is called the acute phase response. Acute phase proteins—these are mostly synthesized by hepatocytes, in which transcription is controlled by cytokines including interleukin 1, interleukin 6, and tumour necrosis factor. The circulating concentrations of complement proteins and clotting factors increase by up to 50 to 100%; some of the proteinase inhibitors and α‎1-acid glycoprotein can increase three- to fivefold; but C-reactive protein (CRP) and serum amyloid A protein (an apolipoprotein of high-density lipoprotein particles) are unique in that their concentrations can change by more than 1000-fold. C-reactive protein—this consists of five identical, nonglycosylated, noncovalently associated polypeptide subunits. It binds to autologous and extrinsic materials which contain phosphocholine, including bacteria and their products. Ligand-bound CRP activates the classical complement pathway and triggers the inflammatory and opsonizing activities of the complement system, thereby contributing to innate host resistance to pneumococci and probably to recognition and safe ‘scavenging’ of cellular debris. Clinical features—(1) determination of CRP in serum or plasma is the most useful marker of the acute phase response in most inflammatory and tissue damaging conditions. (2) Acute phase proteins may be harmful in some circumstances. Sustained increased production of serum amyloid A protein can lead to the deposition of AA-type, reactive systemic amyloid.