7. Antibody: Function Flashcards
What are the two main functions of the antibody?
- Recognise antigen
2. Eliminate pathogen
How do antibodies usually eliminate pathogens?
By linking to other immune system components when it has bound antigen - constant region of heavy chain important for this
What are the Fab and Fc regions of the antibody?
Fab are the flippy floppy arms, and the Fc is the sticky wicky tail
What are the two different situations antibodies will be found?
A transmembrane version on the surface of B cells OR released from plasma cells and distributed throughout the whole body
What is the epitope?
The exact point at which the amino acids of the CDR make contact with the antigen (epitope is on the antigen)
How does the antibody make contact with the antigen?
CDRs on the Fab arm recognise the epitope on the surface of the antigen and make contact (close contact necessary)
An Antibody will always recognise the SAME area of the antigen (footprint region) but different antibodies will recognise different epitopes
What is the usual footprint of the antigen (size of the epitope)?
2.5nm^2
Define the antigen footprint
The size of the epitope of the antigen
How does the number of epitopes vary with antigen size?
Number of epitopes increases with antigen size
What is the difference between discontinuous and continuous epitopes of antigen proteins?
Discontinuous: Amino acids recognised not in continuous linear sequence (85% of these require natural shape of antigen) - close together in tertiary sequence but far apart in primary sequence - some of these also known as conformation epitopes
Continuous: amino acid recognised as continuous linear sequence
How many amino acids are involved in epitopes? Thus how many amino acids does each CDR bind?
15-20 AA on epitopes, so each CDR binds 2-3 AA (not all AA contribute equally to binding though)
What is true of all the antigen-antibody binding forces?
They are all NON-covalent
What are the 4 antigen-antibody binding forces? Order from closest binding to loosest binding
hydrophobic-VDW-electrostatic-H-bonding
what is the equation for the law of mass action with respect to antigen/antibody? How is this applicable?
K = [AbAg]/[Ab][Ag]
As all the AbAg forces are non-covalent Ab will bind Ag then let go then bind again etc so there will always be some Ab/Ag unbound and some AbAg
Why has evolution developed Antibody with two antigen binding arms (fab fragments)?
More antigen binding arms increase the strength of binding and also allow multivalent binding
What is avidity?
The accumulated strength of multiple affinities of individual non-covalent binding interactions (e.g. AbAg)
What is the increase in affinity equilibrium constant caused by multivalence?
1000 fold increase for IgG, 10^7 fold increase for IgM (can link 5 antibodies together so have 10 Ag binding arms)
What is the bonus effect of multivalency?
Can link two antigens using each arm
How do IgM antibodies and IgG antibodies vary in amounts between the primary and secondary immune responses?
IgM: do not vary in primary and secondary antibody response
IgG: dominant in primary and massively increased in secondary
This is due to class switching (a similar process to VDJ)
How are secreted antibodies generated from transmembrane antibodies?
Determined at immunoglobulin level. When making secrete antibodies, the last two exons of the C-mu region are not encoded so no stretch of hydrophobic AA encoded therefore nothing to hold it in the membrane so antibody is releases/secreted
What are the three methods antibodies may work on their own to hinder pathogens?
- Bind to bacterial toxin and prevent it entering/poisoning cells
- Block viruses binding to cellular receptors
- Block adherence of bacteria to mucosal surfaces (Ab bind instead)
What is the most common role of antibodies?
Link other parts of the immune system to the pathogen
Which is the best type of antibody at neutralising pathogens?
IgM due to the 10 Ag binding arms of the pentamer
How do Red blood cells (RBCs) interact with Ab-Ag-complement complexes
RBCs have CR1 complement receptors which recognise the C3B of complement when it is bound to an Ag-Ab complex and take the complex to the liver/spleen where they are phagocytosed
How are antibodies involved in opsonisation?
Phagocytic cells have Fc receptors on their surface, these bind the Fc region of the antibody (e.g. Fc gamma Receptors bind IgG)
What are the 5 main antibody bridge roles?
- Clearance of immune complexes (RBCs)
- Direct neutralisation
- Immune complex formation, opsonisation
- Complement activation
- Sensitisation of target cells
Which antibody is particularly good at opsonisation?
IgG
Which antibodies are specialised in activating complement? Which complement pathway do they activate?
IgM and IgG activate through classical pathway (form MAC)
What is the process of antibody dependent cell mediated cytotoxicity? What antibody is best at this?
Mediated by killer cells, requires cell with Fc receptor and ability to produce toxic molecules, occurs in pathogen which are too large to be phagocytosed, IgG is best at this
What antibody is responsible for the inflammatory response? How?
IgE, may causes Mast cell to release inflammatory mediators (mast cells have Fc epsilon receptor - binds IgE)
How may B cells act as professional antigen presenting cells?
Use antibody to grab antigen, internalise, degrade into peptides, place onto surface of B cell in the MHCII to activate T cells
What is the function of IgM and IgD?
Antigen receptors on naive B cells, each behave as antibody monomers.
IgM when released from plasma cells forms pentamer which acts in blood produced in the primary immune response and good at activating complement and causing agglutination
What is the role of IgG?
Most concentrated of all Ig’s (GAMDE). Four subclasses:
IgG1 - activates complement, enhances phagocytosis, foetal protection (only class that can cross placenta - so maternal antibodies provided for foetus)
IgG2 - activates complement
IgG3 - activates complement, enhances phagocytosis
IgG4 - can cross the placenta
What is the role of IgA?
Two subclasses, 2nd most dominant, not present in circulation so protects mucosal surfaces
What is the role of IgD?
Constitutes the antigen receptor on naive B cells (along with IgM)
What is the role of IgE?
Triggers release of inflammatory mediators from mast cells when in the presence of antigen, this is important in protection against parasitic worms and in allergy
