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The Complement System
1. Introduction
2. Activation
3. Regulation
4. Complement receptors
5. Functions
1. Introduction to the complement system
complement system
It is an ancient but important defence system
Approximate 30 proteins forming one of the major defence
systems in the body, that are normally inactive but become
activated under certain circumstances
• Accounting for ~10% of serum proteins, C3 is the
predominant component.
• Sensitive to physical and chemical factors, the complement
activity can be inactivated by incubation at 56℃for 30 min
• Most complement components are produced by
hepatocytes and macrophages
Complement components
2. Complement activation
Complement doesn’t function until being activated
2.1. Activation pathways
2.1.1 Classical
2.1.2. MBL
2.1.3. Alternative
2.2. Terminal stage of activation
All 3 pathways have a terminal activation in
common — formation of the MAC
2.1. Activation pathways
There are 3 pathways - differ in how C3 convertase is generated
• Classical pathway - activation by antibody-antigen complexes
(adaptive)
• MBL pathway - activation by carbohydrates present on
microbes in absence of Ig (innate)
• Alternative pathway - activation by microbes’ ‘protected’
surface in absence of Ig (innate)
2.1.1. Classical pathways of complement activation
The classical pathway is activated by immune complexes
This activation pathway begins with the C1q binding to the Fc part of
antibody (IgM and IgG) attached to antigens.
Classical pathway starts from the C1q binding to
antigen-antibody complex
Activation of complement by IgG and IgM antibodies
Classical pathway of complement activation
Classical pathways of complement activation
• After formation of antigen-antibody complex, and C1q binds to
the Fc fragment of the antibody
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