Acute-phase proteins are a class of proteins whose plasma concentrations increase (positive acute-phase proteins) or decrease (negative acute-phase proteins) in response to inflammation. This response is called the acute-phase reaction (also called acute-phase response).
In response to injury, local inflammatory cells (neutrophil granulocytes and macrophages) secrete a number of cytokines into the bloodstream, most notable of which are the interleukins IL-1, IL-6 and IL-8, and TNF-α.
The liver responds by producing a large number of acute-phase reactants. At the same time, the production of a number of other proteins is reduced; these are, therefore, referred to as "negative" acute-phase reactants. As such, increased acute phase proteins from the liver may also contribute to the promotion of sepsis.
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Positive acute-phase proteins serve different physiological functions for the immune system. Some act to destroy or inhibit growth of microbes, e.g., C-reactive protein, mannose-binding protein[1], complement factors, ferritin, ceruloplasmin, Serum amyloid A and haptoglobin. Others give negative feedback on the inflammatory response, e.g. serpins. Alpha 2-macroglobulin and coagulation factors affect coagulation, mainly stimulating it. This pro-coagulant effect may limit infection by trapping pathogens in local blood clots. Also, some products of the coagulation system can contribute to the innate immune system by their ability to increase vascular permeability and act as chemotactic agents for phagocytic cells.
| Protein | Immune system function |
|---|---|
| C-reactive protein | Opsonin on microbes [2] |
| Serum amyloid P component | Opsonin |
| Serum amyloid A |
|
| Complement factors | Opsonization, lysis and clumping of target cells. Chemotaxis |
| Mannan-binding lectin | Mannan-binding lectin pathway of complement activation |
| Fibrinogen, prothrombin, factor VIII, von Willebrand factor |
Coagulation factors, trapping invading microbes in blood clots. Some cause chemotaxis |
| Plasminogen | Degradation of blood clots |
| Alpha 2-macroglobulin |
|
| Ferritin | Binding iron, inhibiting microbe iron uptake |
| Hepcidin[4] | Stimulates the internalization of ferroportin, preventing release of iron bound by ferritin within intestinal enterocytes and macrophages |
| Ceruloplasmin | Oxidizes iron, facilitating for ferritin, inhibiting microbe iron uptake |
| Haptoglobin | Binds hemoglobin, inhibiting microbe iron uptake |
| Orosomucoid (Alpha-1-acid glycoprotein, AGP) |
Steroid carrier |
| Alpha 1-antitrypsin | Serpin, downregulates inflammation |
| Alpha 1-antichymotrypsin | Serpin, downregulates inflammation |
"Negative" acute-phase proteins decrease in inflammation. Examples include albumin,[5] transferrin,[5] transthyretin,[5] retinol-binding protein, antithrombin, transcortin. The decrease of such proteins may be used as markers of inflammation. The physiological role of decreased synthesis of such proteins is generally to save amino acids for producing "positive" acute-phase proteins more efficiently. Theoretically, a decrease in transferrin could additionally be decreased by an upregulation of transferrin receptors, but the latter does not appear to change with inflammation.[6]
Measurement of acute-phase proteins, especially C-reactive protein, is a useful marker of inflammation in both medical and veterinary clinical pathology. It correlates with the erythrocyte sedimentation rate (ESR).
They may also indicate liver failure [7]
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