Inflammatory Markers and Acute Phase Response
The acute phase response is the coordinated systemic reaction to infection, tissue injury, and inflammation, in which the liver sharply changes its output of a set of plasma proteins. Measuring these acute-phase proteins and the cytokines that drive them gives clinicians objective, quantitative biomarkers of inflammation, and these tests are among the most frequently ordered in clinical chemistry.
Definition
The acute phase response is a stereotyped systemic change in the concentrations of plasma proteins (acute-phase reactants) produced largely by hepatocytes in response to inflammatory cytokines; inflammatory markers are the laboratory analytes that quantify this response.
Scope
This area orients the reader to the biology of the acute phase response and to the laboratory markers used to detect and follow inflammation: C-reactive protein and high-sensitivity CRP, procalcitonin, the erythrocyte sedimentation rate, the cytokine and interleukin signals that initiate the response, and complement activation. It treats these as analytes and reference concepts in clinical biochemistry, not as a guide to diagnosis or therapy.
Sub-topics
Core questions
- What cytokine signals trigger the hepatic acute phase response?
- Which plasma proteins rise (positive reactants) and which fall (negative reactants) during inflammation?
- How do different inflammatory markers differ in kinetics, specificity, and clinical use?
- How is a non-specific marker of inflammation interpreted alongside the clinical picture?
Key concepts
- Acute-phase reactants (positive and negative)
- Cytokine-driven hepatic protein synthesis
- Interleukin-6 as a principal acute-phase mediator
- Marker kinetics (onset, peak, half-life)
- Sensitivity versus specificity of inflammatory markers
- Negative acute-phase proteins (albumin, transferrin)
Mechanisms
Tissue injury and infection activate innate immune cells that release pro-inflammatory cytokines, chiefly interleukin-6, interleukin-1, and tumour necrosis factor. These cytokines act on hepatocytes to reprogram protein synthesis: positive acute-phase reactants such as C-reactive protein, serum amyloid A, fibrinogen, haptoglobin, and complement proteins increase, while negative reactants such as albumin and transferrin decrease. The magnitude and time course differ by protein, so CRP rises within hours, the sedimentation rate changes more slowly through altered fibrinogen, and procalcitonin responds preferentially to bacterial stimuli. This shared cytokine-to-liver axis is why a single inflammatory stimulus moves many markers together.
Clinical relevance
Inflammatory markers are widely used to detect inflammation, gauge its intensity, and follow its course over time, and they support evidence appraisal across infectious, rheumatologic, and cardiovascular medicine. They are non-specific signals that describe a biological state; their interpretation depends on the clinical context, and this reference entry does not provide diagnostic thresholds or treatment direction.
Epidemiology
Acute-phase reactant tests are among the highest-volume assays in clinical laboratories worldwide because inflammation accompanies a vast range of conditions. The same markers also serve as population-level indicators in research, for example high-sensitivity CRP in cardiovascular risk studies.
Evidence & guidelines
The framework for the acute phase response and its mediators is set out in widely cited reviews (Gabay & Kushner, 1999; Medzhitov, 2008), and individual markers are addressed in their own topic entries with the relevant clinical-laboratory literature. This area page is an orienting overview rather than a guideline summary.
History
Recognition that inflammation produces a systemic plasma-protein response grew through the twentieth century, beginning with the 1930 discovery of C-reactive protein and broadening as the roles of fibrinogen, serum amyloid A, and complement were defined. The identification of interleukin-6 and related cytokines as the hepatic signals integrated these observations into the modern concept of a cytokine-driven acute phase response, consolidated in late-twentieth-century reviews.
Key figures
- Mark Pepys
- Irving Kushner
- Cem Gabay
- Ruslan Medzhitov
Related topics
Seminal works
- gabay-kushner-1999
- pepys-hirschfield-2003
- medzhitov-2008
Frequently asked questions
- What is the acute phase response?
- It is the systemic change in plasma protein concentrations driven by inflammatory cytokines acting on the liver, in which some proteins (such as CRP) rise and others (such as albumin) fall in response to infection or tissue injury.
- Are inflammatory markers specific for a particular disease?
- Most are not; markers such as CRP and the sedimentation rate indicate that inflammation is present and roughly how intense it is, but not its cause, so they are interpreted together with the clinical picture.