5. Functions of complement system
• Lysis of pathogens
• C3 convertase is a product of all the pathways triggering the complement
cascade and is responsible for converting factor C3 into C3a and C3b.
C3b binds to antigens on the pathogen, which stimulates neutrophils and
macrophages to phagocytose pathogens – this is called opsonisation.
6. Lysis of Pathogens
• Lysis of pathogens is facilitated by the formation of the membrane
attack complex (MAC). C3 convertase is vital to the production of the
MAC because it generates C3a and C3b. C3b combines with other
factors to produce C5 convertase, an enzyme which converts factor
C5 to C5a and C5b.
• C5b combines with several factors to produce the MAC. The MAC
ruptures the bacterial cell membrane, allowing fluid to enter the
bacteria and causing cell lysis. However, because they possess a
cell wall, gram positive bacteria and fungi do not swell and hence
cannot be lysed by the complement system.
• The production of C5a by C5 convertase attracts neutrophils
and macrophages to the site of infection and causes
extravasation of leucocytes from capillaries to tissues. C3a is
another complement component that acts as a chemotaxin.
• C3a, C4a and C5a are the complement components
responsible for causing inflammation. They bind to mast cells
and basophils to cause degranulation. The histamine and
serotonin released increase vascular permeability. C3a, C4a
and C5a also promote synthesis of pro-inflammatory cytokines.