Acute phase proteins

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, infection, injury, and some other stress conditions. They are primarily produced by the liver and are a key component of the acute phase response, a complex systemic early-defense system activated by trauma, infection, stress, neoplasia, and inflammation.

Overview

The acute phase response is an early, non-specific reaction to disturbances in the body's homeostasis. This response is characterized by alterations in the concentrations of acute phase proteins in the bloodstream, which serve various functions in the immune response. Positive acute phase proteins, such as C-reactive protein (CRP), serum amyloid A (SAA), fibrinogen, haptoglobin, and α1-antitrypsin, typically increase in concentration. Conversely, negative acute phase proteins, like albumin and transferrin, decrease in concentration during the acute phase response.

Function

Acute phase proteins play several roles in the immune response, including the restoration of homeostasis, direct defense against pathogens, and modulation of the immune system. For example, CRP can bind to the surface of dead or dying cells and some types of bacteria, targeting them for destruction by phagocytes in a process known as opsonization. Fibrinogen, another acute phase protein, contributes to blood clot formation, thereby preventing the spread of infection.

Regulation

The production of acute phase proteins is mainly regulated by cytokines, signaling proteins released by cells of the immune system. Key cytokines involved in this process include interleukin-6 (IL-6), interleukin-1 (IL-1), and tumor necrosis factor-alpha (TNF-α). These cytokines stimulate the liver to produce and secrete acute phase proteins into the bloodstream.

Clinical Significance

Measurement of acute phase proteins, particularly CRP, is a useful marker for detecting inflammation, monitoring the progression of disease, or evaluating the effectiveness of treatment. Elevated levels of CRP and other acute phase proteins can indicate the presence of an inflammatory condition, such as rheumatoid arthritis, inflammatory bowel disease, or bacterial infections.

See Also

• Inflammation
• Immune system
• Cytokine
• Liver