Table 1:

Activators of the three pathways of complement

Pathway	Triggers	References
Classical pathway (CP)	Antibodies (immunoglobulins, Fc portion of IgG, especially IgG1 and IgG3, or IgM) binding to antigens, negatively-charged molecules (DNA, lipopolysaccharides, and heparin) Pentraxins (C-reactive protein [C-RP] and pentraxin-3) that recognize pathogen-associated molecular patterns (PAMPS, e.g., viral proteins) Certain phospholipids (amyloid)	1
Lectin pathway (LP)	Surface-linked carbohydrates (lipopolysaccharides, phosphatidylserine, and immunoglobulin) Acetyl groups on pathogens or damaged self-tissue	2
Alternative pathway (AP)	Hydrolysis (of the thioester bond in C3), bacteria or damaged cells	3