Lectin Pathway Flashcards
Does the Lectin pathway involve Ab?
No
How is the activation of the lectin apthway mediated?
By the surface of the pathogen
What does Lectin use instead of C1q to bind?
Mannose binding Lectin (MBL) or Ficolins
What is Lectin?
A protein that binds sugars
What do MBL and Ficolins bind to?
Specific carbohydrate residues on the pathogen.
What are the C1r and C1s equivalents?
MASP-1 and MASP-2
Once MBL or Ficolin bind to the carbohydrate what happens next?
C4 is cleaved
C4b binds C2 and then what happens?
C2 is cleaved by MBL or Ficolin - leaving C4bC2a on target surface (C3 convertase)
What two things does C3 do?
Cleavage of C3 by MBL or Ficolin, C3b binds target surface (opsonin)
Some C3b binds C4bC2a forming C4bC2aC3b (C5 convertase)
Alternate Pathway does not involve any initiator molecules. What happens instead?
A small amount of curculating C3 spont. activate - “tick-over”
“Tick-Over”
Spont. activation of C3
How is activated C3b inactivated?
hydrolysis (binds water)
What can C3b on an activator surface bind?
Factor B
What cleaves Factor B?
Factor D
What results when Factor D cleaves Factor B?
2 fragments
Ba and Bb
What is the alternate pathway C3 convertase?
C3bBb
What does the C3 convertase facilitate?
Large scale cleavage of C3 producing more C3b
What does the additional C3b do?
Binds to the C3bBb to form C3bBbC3b (C5 convertase)
Describe the alternate pathway
- C3b deposited by classical or lectin pathway
- C3b finds Factor B
- Factor D cleaves Factor B into Ba and Bb
- Ba is released, Bb binds to C3b and makes the C3 convertase C3bBb
- C3bBb can cleave more C3b and make C3bBbC3b (C5 convertase)
What factor stabilizes C3bBb activity?
Factor P
What factor prevents C3b from binding Factor B?
Factor H
Molecules in classical:
C1-C4b-C2a-C3convertase(C4bC2a)-C3b-C4bC2aC3b (C5convertase)-C5b
Molecules in alternate:
C3b-Bb-C3bBb(C3convertase)-C3bBbC3b(C5convertase)-C5b
Molecules in Lectin:
MBL/Ficolin-C4b-C2a-C4bC2a(C3convertase)-C3b-C4bC2aC3b(C5convertase)-C5b
