The Complement System (activating antibodies and beyond) Flashcards
what did richard friedrich johannes pfeiffer work with?
Working with Cholera bacilli
what did richard friedrich johannes pfeiffer note?
noted that guinea pig
anticholera serum lysed cells
richard friedrich johannes pfeiffer heated …
… anti serum at 56 deg C for 30 mins negated this acitvity
addition of fresh serum from control guinea pig restored…
… activity
richard friedrich johannes pfeiffer experiment conclusion?
Antibodies to Cholera, plus a heat-labile component present in the ‘antisera’ and ‘normal’ serum were required for lysis
Later Ehrich classified this
component as
Complement
What is Complement?
A group of serum proteins that act in a coordinated manner to exert their biological (immune) effect
Serum concentration of Complement DOES…
… NOT increase after immunisation
the complement system is a […] process between components of different pathways
the complement system is a DYNAMIC process between components of different pathways
how do complement proteins work?
- complement proteins insert themselves into the membrane of pathogens, creating a pore.
- water and ions enter the cell
- cell swells and lyses
what does the classical pathway require?
Requires antigen/antibody complexes for initiation
(immunoglobulins are potent activators of
Complement; Ig Gand IgM)
the alternative pathway is antibody …
independent.
in the alternative pathway, the pathogen surface provides a…
biochemical environment conducive with Complement initiation
what happens in the lectin pathway?
Mannose binding lectin binds to pathogen surface promotes initiation process
What does the classical pathway involve
Involves antibody (or c-reactive protein) binding to specific epitope on
antigen (pathogen)surface
The classical pathway involves the orderly activation of?
Orderly activation of nine major protein components (C1– C9)
some components in the classical pathway cascade are…
… enzymes
Amplification and activation of large amounts of complement from a small signal is…
… possible
the classical pathway is highly…
… regulated; each step is highly regulated
when does the classical pathway culminate?
Culminates in the C9 polymerisation around
the C5b678 complex -> membrane attack complex (perforin-like structure in pathogen
cell membrane)
C1 split into its 3 compoments…
C1q - 400 kDa
C1r - 95 kDa
C1s - 85 kDa
C1r and C1s are a…
… heterodimer
what kind of activity does C1s have?
C1s has esterolytic and
proteolytic activity that acts on the next component in the cascade = C4
activation of classical pathway requires the binding of…
… C1q to C1q specific receptors on Fc regions
of Ig (at least 2 IgG cross-linking or
at least 2 regions on IgM).
Activation requires the binding of C1q
to C1qspecific receptors on Fc regions
of Ig (at least 2 IgG cross-linking or
at least 2 regions on IgM).
What does this cause?
conformational change in C1q
- activates C1r / C1s
what causes activation of C1q ?
Immunoglobulin (IgG and IgM) activate C1q– at least two interaction points required.
Agents that donotactivate C1q
1) Hapten or monovalent antigen
2) Some Ig’s–IgA and IgE (as these do not have C1q receptors in their Fc region
IgG4 also lacks C1q receptors and is the only IgG that does not activate C1q
What does C1s activate ?
C1s activates the next component-C4
size of C4 ?
180 kDa
What is synthesised by…
liver & macrophages on activation
C4 split into what?
split into C4a and C4b (the larger fragment)
what does C4b bind to?
binds to pathogen cell membrane next to Ab-Ag complex and attaches next to component C2
size of C2 ?
115 kDa
what is C2 cleaved by?
by the action of C1s and C4b
C2 when cleaved becomes…
…C2a
C2a remains associated with…
C4b
C2a remains associated with C4b, which is the…
… C4b2a complex.
what kind of complex is C4b2a ?
an active enzyme complex
what is C4b2a also known as…
… C3/5 convertase
what is C3 ?
C3 is aβ-globulin (180 kDa) secreted by liver & macrophages as pro-C3
Activation of pro-C3 by…
… C3 convertase (cleavage)
pro-C3 cleaved by C3 convertase, which splits into…
… its components which are:
C3a (smaller fragment - remains in serum)
C3b (larger fragment attaches to pathogen cell membrane)
C3b then attaches to…
… target cell membrane
Once C3b attaches to target cell membrane, it is then called …
… C5 convertase
Activation of C3b initiates the
formation of the membrane attack complex–assembly and activation of components C5- C9
Activation of C3b (C5 convertase), causes…
… C5 to split into C5a (small fragment) and C5b
and also activation of C6 and C7, resulting in formation of C5b67 complex.
C5b67 complex associates with…
… the cell membrane and focuses the activity of c8 and c9
wat causes activation of C8 and C9 ?
C5b67 complex
C8 binds to the …
… C5b67 complex, forming the C5b678 complex
C5b678 complex interacts with
cell membrane, creating small pores, resulting in lysis
C5b678 complex directs…
… C9 function (tells C9 where to bind). Then polymerisation creating a circular perforin-like molecule around the C5b678 complex.
C5b678 complex directs C9 function (tells C9 where to bind). Then polymerisation creating a circular perforin-like molecule around the C5b678 complex.
This causes …
… formation of transmembrane channels and disruption of osmotic potential, causing ion flux, cell swelling, and membrane permeability, causing cell lysis.
what is required for C1 cross-linking?
IgG / IgM
There are manyinitiatorsof the alternative pathway (does not require…
… antibody/antigen complex and C1 activation)
How is complement activated without antibodies?
a) Mechanism of spontaneous C3 cleavage and C3b binding to pathogen
b) Mechanism involving factor B
Biological Properties of Complement?
1) Cell lysis
2) Anaphlatoxins (major activators; C3a, C5a and C4a weakly)
3) Chemotaxis
4) Immune adherence
5) Opsonisation
Define Anaphlatoxins
Ananaphlatoxin is a substance that induces mast cell / basophil degranulation– histamine release
Histamine increases
capillary permeability / contraction of smooth
muscle associated with anaphylaxis and other allergic reactions
what happens in chemotaxis?
Attracts phagocytic cells from an area of lesser concentration (of C5a) to area of higher concentration (of C5a)
C5a has a potent effect on neutrophils which express C5aR
Neutrophil chemotaxis in response to
… C5a ligation with the C5a-receptor (C5aR) expressed on the surface of neutrophils.
immune adherence is a major property of…
… C3b
when does immune adherence occur?
Occurs when a particular Ag coated with Ab in the presence of complement, mediates adherence to various surfaces (eg: blood
vessels)
what does immune adherence do?
immobilises the pathogen -> target for phagocytes
opsonisation improves the liklihood of…
… phagocytosis occurring
Mannose-binding protein-associated Serine Protease-2 (MASP-2) has homology to…
… C1and similar biological activity
Unlike C1, MASP-2 doesn’t bind…
… Cq specific receptors on FC (thus is does not require Ag : Ag complex for activation)
MASP-2 binds…
… mannose patterns on pathogen surface -> activated.
Describe the Lectin Pathway.
1) Activated MASP-2 cleaves C4 to C4a and C4b. Some C4b binds covalently to the microbial surface.
2) Activated MASP-2 also cleaves C2 to C2a and C2b.
3) C2a binds to surface C4b forming the classical C3 convertase, C4b2a.
4) C4b2a binds C3 and cleaves it to C3a and C3b binds covalently to the microbial surface.
order of pathways
FIrst to act = alternative pathway
Second to act = Lectin pathway
Third to act = classical pathway
all lead to complement activation
what happens after complement activation?
cleavage of c3 to c3b.
c3b covalently bound to surface components of pathogen.
cleavage of c3 to c3b.
c3b covalently bound to surface components of pathogen.
what happens after?
- Recruitment of inflammatory cells
- Opsonization of pathogens, facilitating uptake and killing by phagocytes
- Perforation of pathogen cell membranes
all lead to death of pathogen.
