Acute phase proteins are a class of proteins that are synthetized in the liver in response to inflammation. This response is called the acute phase reaction.

In response to injury, local inflammatory cells (neutrophil granulocytes and macrophages) secrete a number of cytokines into the bloodstream, most notably the interleukins IL-1, IL-6 and IL-11 and TNF-alpha.

The liver responds by producing a large number of acute phase reactants, most notably:

• C-reactive protein
• Alpha 1-antitrypsin
• Alpha 1-antichymotrypsin
• Alpha 2-macroglobulin
• some coagulation factors (Fibrinogen, prothrombin, factor VIII, von Willebrand factor, plasminogen)
• Complement factors
• Serum amyloid protein (see amyloid)