Greg - Introduction and Complement Flashcards
Define the complement system
The complement system is a biological cascade made up of multiple serum proteins that is part of the innate immune system
What is serum?
The liquid from clotted blood
What is plasma?
The liquid part of nonclotted blood
How was complement discovered?
Serum was taken from a patient and mixed with bacteria
It was observed that the bacteria were destroyed
However if you heat the serum and then carry out the experiment the bacteria are not destroyed, therefore it can be said that a heat-labile component in serum has the ability to lyse bacteria (complement)
On a electrophoresis graph, where is complement found?
Complement is found in the beta globulin region
What percentage of the globulin fraction of serum is made of complemet?
5%
What causes the levels of complement to increase?
An immune response i.e. infection
In an electrophoresis graph where are the antibodies found?
Found in gamma globulin region
What are the four main functions of complement
Formation of MAC
Opsonisation
Disposal of immune complexes and apoptotic cells
Activation of immune responses e.g. vascular permeability, chemotaxis for phagocytes and degranulation
What is the main detrimental role of complement
Inappropriate inflammation
Give an example of a disease caused by inappropriate inflammation
Systemic lupus is caused by an over active complement system and it causes tissue damage
How does complement cause lysis of pathogens?
(4)
Complement is deposited on the surface of pathogen
Complement forms the membrane attack complex (MAC)
This punches holes in the surface of the cell
The cell swells and bursts by osmosis
How do complement proteins act as opsins
(3)
Complement proteins bind to pathogens and mark them for phagocytosis
The phagocytes express receptors such as CR1 which allows the cell to recognise complement attached to a pathogen
When the receptor meets the complement there is a conformational change and the pathogen is phagocytosed
How does complement activate the inflammatory response?
If complement binds to the receptors found on granulocytes it causes these cells to degranulate e.g. mast cells/gatekeeper cells releasing histamine causing inflammation and vasodilation
How do complement proteins clear immune complexes
Complement marks antibody-antigen complexes for phagocytosis in the liver and spleen
What does CR1 stand for?
Complement Receptor 1
How many proteins make up complement?
30+
What cells make complement?
Hepatocytes (mostly produced here)
Epithelial cells of the gastrointestinal and genitourinary tracts
Monocytes/macrophages
What type of proteins are complement proteins?
Acute phase proteins
Zymogens
Complement proteins are zymogens, what does this mean?
Zymogens are inactive forms of enzymes
It means complement needs to be activated before working
How are complement proteins activated?
Proteolytic cleavage (the chopping off of part of the protein)
How are complement proteins numbered?
C1, C2, C3 …. C9
After cleavage how are the components of complement named?
Usually the larger component gets named B and the smaller A
e.g. C3a and C3b
However for C2, C2a is the larger piece and C2b is the smaller piece
What does the b part of complement proteins usually do?
b is usually the membrane-binding fragment
How do you write an activated component?
You overline the components
What are convertases?
Complexes with proteolytic activity e.g. C3 convertase cleaves C3 into C3a and C3b
How many complement pathways are there?
Three:
- classical pathway
- alternative pathway
- mannose-binding lectin pathway
What is the endpoint of all three complement pathways?
The formation of MAC
How does the classical pathway differ from the other two pathways?
The classical pathway is antibody dependent
Why are antibodies needed for the classical pathway?
Complement fixation is needed and this only happens because there was an antibody there first and then complement came along -> in order for complement to stick to the surface of something there needs to be an antibody there first
What antibodies initiate the classical pathway?
IgM and IgG
Which subclasses of IgG initiate the pathway?
3/4 subclasses initiate the pathway
Which is better at complement activation, IgG or IgM?
IgM is way better
What is the first type of antibody made after an infection?
IgM is the first antibody secreted by plasma cells they may switch to different types such as IgG after
What is the first molecule involved in the classical pathway?
C1q
What does C1q bind with during the classical pathway?
2x C1r and 2x C1s to form C1qrs
Where is C1q found?
It is found floating around in circulation
What are the two fragments of antibodies?
Fab (fragment with antigen binding) and Fc (fragment that is crystallisable) fragments
How many Fc fragments does C1qrs have to bind to?
C1qrs has to bind to two Fc fragments on antibodies (this is why the classical pathway is complement dependent)
What is the evolutionary point of the classical pathway needing two Fc fragments to bind to on antibodies?
This prevents C1qrs activating in circulation when it accidently bumps into an antibody
It prevents unwanted inflammation
What happens after C1qrs binds to two Fc fragments?
There is a conformational change in shape and one of the C1r’s is activated -> this then activates the other C1r -> this activates both of the C1s s
Where exactly on the Fc fragment does complement bind?
The Ch2 domain
Why is IgM much better at activated C1qrs than IgG?
IgM is a pentamer while IgG is a monomer
What happens after C1s s are activated?
C1s can now cleave C4 and C2 to give C4a, C4b, C2a and C2b
What does C4b do?
It sticks down on to the surface of the pathogen
What does C2a do?
It sticks down to the surface of the pathogen along with C4b to form C42a
What happens after C4b2a is formed?
C4b2a is a C3 convertase
=> C3 is cleaved into C3a and C3b
What happens after C3 is cleaved
C3a floats away
C3b sticks to the pathogen surface
C4b2a3b is formed
What happens after C4b2a3b is formed?
C4b2a3b is a c5 convertase
=> c5 is split into C5a and C5b
C5b binds to C6
What happens when C5b binds to C6
This initiates the formation of the membrane attack complex
How is the MAC formed
C5b, C6 and C7 bind together to form C5b67
C8 then joins to form C5b678
Many C9 molecules then join to create a pore and form C5b6789
How does the Mannose binding lectin pathway work?
MBL attaches to mannose/frucose containing polysaccharides on bacteria
MBL is then bound by MASP-1 and MASP-2
What does MASP stand for?
MBL associated serine proteases
How many subunits are there to MBL?
6
When can the MBL pathway begin?
Day one of infection
How does the alternative pathway work?
C3 can spontaneously split
Write a note on the MAC
Membrane attack complex
Best at killing gram negative bacteria - low peptidoglycan
Gram positive are generally resistant to the MAC
What do the Ca s do after they have been released?
They are anaphylatoxins (pro inflammatory molecules)
What do anaphylotoxins do?
Activate basophils and mast cells to degranulate resulting in increased vascular permeability and contraction of smooth muscle cells
What is C3bs function outside of pathway activation?
it acts as an opsonin
What is C4bs function outside of pathway activation?
It acts as an opsonin
List the anaphylatoxins
C5a
C4a
C3a
What are the functions of anaphylatoxins
(4)
Degranulation of mast cells
Smooth muscle contraction
Increased vascular permeability
C5a causes chemotaxis of leukocytes
Write a note on C3 deficiency
Most severe clinical manifestations of complement deficiency
Recurrent bacterial infection and immune complex diseases
Give two examples of conditions that are caused by regulatory protein deficiencies
Hereditary angioedema
Paroxysmal nocturnal haemoglobinuria
Write a note on hereditary angioedema
(3)
Caused by mutations in the C1 inhibitor
Patients experience intermittent episodes of oedema
Oedema can be life threatening if airway becomes blocked
How does C1 inhibitor work?
C1 inhibitor causes the dissociation of C1r and C1s from C1q
This prevents the classical pathway
Write a note on paroxysmal nocturnal haemoglobinuria
(2)
An acquired disorder where a defect in PIGA results in a deficiency in GPI-anchored proteins including CD59
Without this RBCs are unprotected from lysis by complement
How is paroxysmal nocturnal haemoglobinuria treated?
The monoclonal antibody Eculizumab is used
Eculizumab binds C5 and halts the complement cascade
What is the CH50 assay used for?
It demonstrates the ability of serum complement to lyse sheep RBCs coated with antibody
What does a high CH50 mean?
A poor complement activation
