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How Complement System Works (Lectin Pathway)
The lectin pathway is part of the complement system, which is part of the innate immune system. It acts as the first line of defense against microbes entering the body. In the lectin pathway, MBL attaches to mannose on microbes. This activates the complement system by breaking down C4 and C2 proteins to form a C3 convertase, which then breaks down C3 into C3b and C3a. C3b opsonizes the microbe while C3a causes inflammation. C3b then binds to the microbe and C3 convertase to form a C5 convertase, continuing the complement cascade and ultimately forming a membrane attack complex that causes cell lysis of the micro
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How Complement System Works (Lectin Pathway)
- 1. How Complement Works TheLectin Pathway
- 2. Stage 1 • Microbesenter the body through wounds, respiratory means, digestion, etc. Microbes Person
- 3. Stage 2 • Microbesthat have entered the body stimulate the innate immune system (in all age groups) • This acts as the first line of defence Microbes C3 Macrophage Natural Killer Cells Neutrophils MBL C3b C3a
- 4. Stage 3 • Complementis activated and comprises of two pathways in the innate immune system – The alternative pathway – The lectin pathway • The classical pathway is part of the adaptive immune system Microbes C3 MBL C3b C3a C3 C3b C3a C3 C3b C3a
- 5. • In thelectin pathways, the MBL attaches to the mannose on the microbe MBL Stage 4 Microbe Mannose
- 6. Stage 5 a) TheMASP2 on the MBL breaks down the C4 protein into C4b and C4a. C4a leads to inflammation and cell recruitment b) The protein C2 is also broken down into its ‘b’ (major) and ‘a’ (minor) components C4 C4b C4a a) b) C2 C2a C2b
- 7. Stage 6 • The‘b’ components of the C4 and C2 proteins bind together to the microbe to form a C3 convertase MBL Microbe C4b2b C3 convertase
- 8. Stage 7 • Theprotein C3 is broken into its ‘b’ and ‘a’ components by the C3 convertase • The C3a leads to inflammation and cell recruitment • The C3b leads to opsonisation and removal of immune complexes • It is thought that this process happens spontaneously and thus the C3 convertase would catalyse the breakdown C3 C3b C3a
- 9. Stage 8 • TheC3b protein binds to the microbe and the C3 convertase molecule to make a C5 convertase Microbe C3b C3b C4b2b3b C5 convertase
- 10. Stage 9 • C5is broken down into C5b and C5a • The C5a protein leads to inflammation and cell recruitment C5 C5b C5a
- 11. Stage 10 • C6binds to C5b and accepts C7, C7binds to the lipid membrane and C8 then initiates binding and polymerization of C9 • C6, C5b, C7 and C8 are all components of MAC (membrane attack complex) C5b C7 C6 C8 Plasma Membrane
- 12. Stage 11 • C9is another component of MAC • It binds to C5b, C6, C7 and C8, and polymerizes to form membrane pores • MAC leads to cell lysis and death of the microbe through osmotic lysis C5b C7 C6 C8 Plasma Membrane C9 MAC