Complement System
The complement system, specifically through the actions of proteins
C3a and C3b, destroys microbes.
– Comprises inactive proteins that circulate in the blood; of these proteins, the products of C3 cleavage (C3a and C3b) have multifold functions.
- Three pathways lead to C3 cleavage:
– In the
classical pathway, C1 is "fixed" to antibody-antigen complexes, which initiates a cascade of events that lead to C3 cleavage.
– The
alternative pathway is triggered by spontaneously activated C3b.
– The
lectin pathway is triggered when lectins, such as,
mannose-binding lectin, binds microbial sugars and marks them for phagocytosis.
–
C3a has
pro-inflammatory effects; it recruits neutrophils and macrophages.
–
C3b opsonizes microbes, which involves binding to pathogens and marking them for phagocytosis.
–
Membrane Attack Complexes (MAC):
C3b combines with other complement proteins (C5b, C6, C7, C8, and C9) to form a pore in the membrane of the microbe; massive water influx through the MAC lyses the microbe.
