U01.04.012 Acute phase reactants

Acute phase reactants are proteins whose plasma concentrations increase or decrease in response to inflammation. These proteins play a crucial role in the body’s defense mechanisms during injury, infection, or stress.

Positive Acute Phase Reactants (Upregulated)

Reactant	Function
Ferritin	Binds and sequesters iron to inhibit microbial iron scavenging.
Fibrinogen	Acts as a coagulation factor; promotes endothelial repair and correlates with ESR (erythrocyte sedimentation rate).
Serum Amyloid A	Prolonged elevation can lead to amyloidosis.
Hepcidin	Decreases iron absorption by degrading ferroportin and reduces iron release from macrophages, leading to anemia of chronic disease.
C-reactive Protein (CRP)	Functions as an opsonin, fixes complement, and facilitates phagocytosis. It is measured clinically as a nonspecific sign of ongoing inflammation.

Positive Acute Phase Reactants (Upregulated)

Negative Acute Phase Reactants (Downregulated)

Reactant	Function
Albumin	Its reduction conserves amino acids for the synthesis of positive acute phase reactants.
Transferrin	Internalized by macrophages to sequester iron, thereby reducing its availability to pathogens.

Negative Acute Phase Reactants (Downregulated)

Points to Remember

Ferritin helps inhibit microbial iron scavenging by binding and sequestering iron.
Fibrinogen is crucial for coagulation and endothelial repair.
Serum Amyloid A can cause amyloidosis if elevated for long periods.
Hepcidin plays a role in anemia of chronic disease by reducing iron absorption and release.
C-reactive Protein (CRP) is a key marker of inflammation, aiding in phagocytosis.
Albumin levels decrease to conserve amino acids for other acute-phase reactants.
Transferrin sequesters iron within macrophages to limit its availability to pathogens.

USMLE Step 1 Review Series

Curriculum

U01.04.012 Acute phase reactants

Positive Acute Phase Reactants (Upregulated)

Negative Acute Phase Reactants (Downregulated)

Points to Remember

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