U01.04.012 Acute phase reactants

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Learning Objectives

Distinguish between Positive and Negative acute-phase reactants. Master the physiological roles of key proteins like CRP, Hepcidin, and Fibrinogen, and understand how their levels correlate with clinical markers like the ESR and Anemia of Chronic Disease.

1. Positive Acute-Phase Reactants (Upregulated)

Positive reactants are proteins whose serum concentrations increase during inflammation, primarily induced by IL-6. They facilitate the immune response and shield the host from microbial scavengers.

Reactant Primary Function Clinical Significance
C-reactive protein (CRP) Opsonin: fixes complement and aids phagocytosis. Nonspecific marker of ongoing inflammation.
Ferritin Binds and sequesters iron. Inhibits microbial iron scavenging.
Fibrinogen Coagulation factor: promotes endothelial repair. Correlates with ESR.
Hepcidin Degrades ferroportin; ↓ iron absorption and release. Leads to Anemia of Chronic Disease.
Serum Amyloid A Apolipoprotein related to HDL. Prolonged elevation → Secondary Amyloidosis.

2. Negative Acute-Phase Reactants (Downregulated)

Negative reactants are proteins whose serum concentrations decrease during inflammation. This shift allows the liver to preserve amino acids for the synthesis of positive reactants.

Reactant Mechanism / Note Purpose of Decrease
Albumin Primary serum protein for oncotic pressure. Conserves amino acids for positive reactants.
Transferrin Iron transporter protein. Internalized by macrophages to further sequester iron.
Transthyretin Also called Prealbumin. Conserves resources for the acute-phase response.

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3. Specific Diagnostic Reactants

Certain reactants serve as specific indicators for the type of infection or the degree of oxidative stress present in the body.

Reactant Function Clinical Context
Procalcitonin Precursor of calcitonin. Specifically increases in bacterial infections.
Haptoglobin Binds extracellular hemoglobin. Protects against oxidative stress during hemolysis.
Complement Proteins C3, C4. Aids in pathogen destruction and clearance.

 

Activity

High-Yield Mnemonics & Tips:

  • More F’s for More Iron: Ferritin is a positive reactant (UP), while Transferrin is a negative reactant (DOWN). Both work to hide iron from bacteria.
  • Hepcidin: Remember it “hides” the iron. High hepcidin levels degrade ferroportin, leading to the trapped iron seen in anemia of chronic disease.
  • Albumin: This is the most consistent negative reactant. In chronic inflammation, low albumin is almost always present.

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