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How does the complement system damage invading microorganisms?
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Blood sediment in test tubes, showing plasma (clear fluid), red blood cells (red), and the release of hemoglobin into the surrounding plasma (pink).
The complement system damages invading microorganisms primarily through the action of the protein fragment C3b, which is generated when the system is activated. C3b facilitates the elimination of pathogens in two main ways:12
- Membrane Attack Complexes: C3b triggers a sequence of reactions that leads to the formation of a membrane attack complex (composed of complement proteins C5 through C9). This structure inserts itself into the pathogen's membrane, creating holes that cause the cell's contents to leak out, ultimately resulting in cell death via lysis.12
- Opsonization: C3b molecules coat the surface of the microorganism, a process known as opsonization. This coating makes the pathogen highly attractive to phagocytic cells such as macrophages and neutrophils. These cells recognize and bind to the C3b, which stimulates them to ingest and destroy the invader.12
Additionally, smaller fragments released during the activation process, such as C3a, promote inflammation, which helps recruit more immune cells to the site of infection to assist in clearing the microorganisms.12