The effect on the immunology laboratory of the expansion in complement therapeutics
According to Gorbet, spontaneous hydrolysis in alternative pathway of the C3 internal thioester group occurs in a fluid phase which generate C3 · H2O. The hydrolyzed C3 will bind and activate the Factor B and cleave C3 molecule (C3a and C3b).
Three complement pathways were discovered around 1900s.
1. Classical Pathway
- Activated by interaction with antibody sensitized cells.
2. Alternative Pathway
- Activated by the unprompted cleavage of C3 followed by hydrolysis and inactivation of C3b in fluid phase.
3. Mannan-Binding Lectin Pathway
- Activated by binding of mannose-binding lectin (MBL) to microbial surface.
To answer your question, alternative pathway is the complement pathway that is initiated by the spontaneous hydrolysis of C3 molecule.
(Source: Henry's Clinical Diagnosis and Management by Laboratory Methods, 24th Edition)
The alternative pathway (AP) is activated when C3 undergoes spontaneous hydrolysis and forms the initial AP C3 convertase, C3 (H2O), in the presence of Factors B and D, leading to additional C3 cleavage and eventual formation of the AP C3 convertase (C3bBb) and AP C5 convertase (C3bBbC3b).
The alternative pathway of complement activation that is initiated by spontaneous hydrolysis of C3 molecule at high concentration in the serum
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