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complement pathway Classical Pathway, Alternative pathway, Lectin pathway General Pathology

The complement system or complement pathway includes a Group of proteins that circulate in the blood as inactive precursors. After stimulation by antibody complexes or by pathogens, complement proteases can activate other complement precursors, leading to a cascade of reactions that is referred to as “complement activation.” The complement system can enhance the function of antibodies and phagocytes as discussed in this animated video.

Activation

Classical pathway: activated by IgM or IgG complexes binding to the pathogen (via C1)
Alternative pathway: activated directly by pathogen rather than by antigenantibody complexes (via C3)
Lectin pathway (via C1like complex)

Effect

Opsonization: the process of altering bacteria to increase their susceptibility to phagocytosis. C3b and IgG are the two main opsonins for bacteria (via C3b).
Lysis of bacteria (particularly gram‑negative bacteria).
C5–C9 form the membrane attack complex (MAC) → perforation of the cell wall → cell lysis
Activation of mast cells and granulocytes (C3/C4/C5) → anaphylaxis
Chemotaxis of neutrophils → C5a
Clearance of immunecomplexes → C3b

Inhibition

DAF decayaccelerating factor (CD55)
C1 esterase inhibitor → e.g. on red blood cells

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