
Heme is an essential component of hemoglobin, the protein in red blood cells that transports oxygen.
Image Credit: Scientific Frontline
Scientific Frontline: Extended "At a Glance" Summary: Heme-Induced Inflammation
The Core Concept: When red blood cells break down, they release heme, an essential, oxygen-transporting component of hemoglobin. Once outside the red blood cell, free heme acts as a harmful agent that can trigger severe inflammatory reactions and damage tissues and blood vessels.
Key Distinction/Mechanism: Free heme binds to and activates the protein factor XII in the bloodstream, which subsequently activates an inflammatory protein network known as the kallikrein-kinin system. This cascade releases bradykinin, a molecule that dilates blood vessels and increases their permeability, causing fluid to leak into surrounding tissues, resulting in swelling, inflammation, and a drop in blood pressure.
Major Frameworks/Components:
- Hemolysis: The physical breakdown of red blood cells caused by infections, trauma, burns, or mechanical stress (such as from mechanical heart valves).
- Factor XII Activation: The initial protein binding site for free heme that acts as the catalyst for the inflammatory cascade.
- Kallikrein-Kinin System: The specific inflammatory pathway in the blood that is triggered by factor XII.
- Bradykinin Release: The molecular output that directly alters blood vessel permeability.
- C1-Inhibitor Intervention: An existing drug used successfully in experimental models to inhibit the kallikrein-kinin system and block the inflammatory reaction.





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