4 Antibodies Flashcards
Q: What is an antibody?
A: a protein that is produced by B lymphocytes in response to a foreign molecule (antigen), and has the property of binding specifically to that antigen.
Q: What class of proteins are antibodies?
A: immunoglobulins - a large family of soluble glycoproteins
specifically the GAMMA IMMUNOGLOBULIN category
Q: What happens after an antibody binds to an antigen? (3)
A: antibodies initiate secondary effector functions:
Complement Activation
Opsonisation
Cell activation via specific antibody-binding receptors (Fc receptors)
*Q: Explain with the aid of a simple diagram the immunoglobulin molecule.
A:
L
N ————————C
S
H S
N————————————————————-C
S
H S
N————————————————————-C
S
L S
N ————————C
N, C = polypeptide terminals
4 polypeptide chains held together by disulphide bonds
The two heavy chains are identical and the two light chains are identical
SYMMETRICAL
Q: Describe the flexibility of the antibody structure? Allows? (2)
A: the hinge regions can bend
The flexibility allows both antibody Fabs to bind to the antigens as the antigens may be narrowly or widely spaced
allows binding to antigens on separate pathogens-> agglutination
*Q: Identify the antigen-binding site (Fab) and Fc portions of antibody.
A:
L
N ————————C
S
H S
N—————————– ——————————–C
S
H S
N—————————– ——————————-C
S
L S
N ————————C
2 fragments bound to antigens - Fab
1 fragment formed protein crystals - Fc: changes conformation when the antigen is bound and can perform effector functions such as activating complement
Q: Identify the constant and variable portions of antibody.
A:
V(L) C(L)
N ———–|————-C
S
V(h) C(h) S
N————|————————————————-C
S
S
N————|————————————————-C
S
L S
N ———–|————-C
Both light chains and heavy chains can be divided into Variable and Constant regions.
The constant part is the Fc part which doesn’t have to be variable because it does not bind to antigens
Q: Where are the other disulphide bonds within an antibody molecule?
A: Apart from the disulphide bridges which hold the different chains together, each domain has an internal intrachain disulphide bond - these are immunoglobulin domains
Q: What makes a protein part of the immunoglobulin superfamily?
A: if the protein has an Ig domain
Q: What is the cross-reactivity of an antibody?
A: are highly specific but they will sometimes react with other molecules
Q: What is noticed when sequencing the amino acid sequence in terms of variability noticed?
A: variability isn’t evenly distributed.
There are THREE REGIONS which are HYPERVARIABLE and they are called the Complementarity Determining Regions (CDR)
Q: What are CDR?
A: loops of amino acids at the end of the protein- part of the protein that is actually binding to the antigen and determines the specificities
Q: Describe the constant region structure.
A: has a barrel shaped beta-pleated sheet which is held together by the highly conserved internal disulphide bonds
Q: In terms of antibody structure, where are the hypervariable regions?
A: hypervariable regions line up at the end of the V domains
Q: What are the forces involved in antibody-antigen binding? (4) What’s their strength?
A: non-covalent
They have a large number of: Hydrogen Bonds Ionic Bonds Hydrophobic Interactions van der Waals interactions
Individually these bonds are weak but overall they are pretty strong.
*Q: Distinguish between antibody affinity and avidity.
A: Affinity = The strength of the total non-covalent interactions between a single antigen-binding site and a single epitope on an antigen.
Avidity
The overall strength of multiple interactions between an antibody with multiple binding sites and a complex antigen with multiple epitopes
*Q: Is antibody affinity or avidity a better measure of
binding capacity in biological systems? Give an example of high a___.
A: Avidity
Generally, the more binding sites that are bound to antigens, the higher the avidity.
EXAMPLE of high avidity - IgM pentamer with ten binding sites interacting with ten epitopes
Q: Describe antibody cross-reactivity.
A: Antibodies produced in response to one antigen can cross-react and bind to a different antigen of similar structure
*Q: List the immunoglobulin classes. Include their heavy chain type. Include how many CH domains they have.
A: IgG-gamma-3, IgA-alpha-3, IgM-mu-4, IgD-delta-3, IgE-epsilon-4
Q: What are isotypes? (2)
A: immunoglobulin type that is present in everybody
Differ in the constant regions
Q: What are allotypes? (3)
A: immunoglobulin type- allele of the antibody chains found in the individual -> Polymorphic variables
Some have them, others don’t
It can be spread anywhere within the constant regions
Q: How do different classes of antibodies differ? What causes them?
A: differ in the CONSTANT REGIONS OF THEIR HEAVY CHAINS
There are different genes for the different heavy chains in the different classes
Q: Which type of light chain do the classes of antibodies have?
A: All the different classes can either use a kappa or lambda light chain - only one type per antibody because they are symmetrical - either both kappa or both lambda
Q: Which antibody classes have subclasses? What are the subclasses? What are their H chains?
A: IgA- IgA1, IgA2 : H chains= alpha 1 +2
and IgG- IgG1, IgG2, IgG3, IgG4 : H chains= gamma 1-4
Q: Describe IgG. Heavy chain type? Abundancy? Form? Good at? (2)
A: gamma heavy chain
Most ABUNDANT immunoglobulin
Monomer
good at neutralising and good at opsonising
Can move across the placenta
Major activator of the classical complement pathway
Q: How do IgG subclasses vary?
A: Classes show variations in the hinge region-
Some have lots of disulphide bonds and some don’t
They have slightly different effector functions
1 to 4 is numbered based on abundance - 1 being the most abundant
They differ in their ability to activate complement
Q: Which receptors on the phagocyte can bind to IgG?
A: FcgRI
*Q: Describe IgA. Heavy chain type? Abundancy? Form in blood? Form in secretions? Role?
A: alpha heavy chain
Second most abundant immunoglobulin
Blood - Monomer
Secretions - Dimer
Protects mucosal surfaces from bacteria, viruses and protozoa
*Q: What is the structure of secretory IgA?
A: Dimer. They are joined together by a J chain (joining chain) and a secretory Component - helps protect the molecule from being degraded by the enzymes found on mucosal surfaces
SECRETORY COMPONENT IS DERIVED FROM THE POLY-Ig RECEPTOR
*Q: Describe IgM. Heavy chain type? Form? Binding sites? Efficient at? Produced when? What does it activate?
A: Mu heavy chain
Pentameric- 5 monomers joined by the J chain and disulphide bonds
Multiple binding sites compensate for low affinity (high avidity)
Efficient at AGGLUTINATION
First Ig produced after exposure to antigen - PRIMARY ANTIBODY RESPONSE
Activates complement
*Q: Describe IgD. Heavy chain type? Serum concentration? When are they expressed? Role?
A: delta heavy chain
LOW serum concentrations
Surface IgD expressed early in B cell development
Involved in B cell development and activation
*Q: Describe IgE. Heavy chain type? Serum concentration? Produced when? Binds to? Function?
A: epsilon heavy chain
LOW concentration in blood
Produced in response to parasitic infections and in allergic diseases
Binds to high affinity Fc receptors of mast cells and basophils
Cross-linking by antigen triggers mast cell activation and histamine release
IgE coats mast cells and bind to the Fc receptor on mast cells
The allergen will bind to the IgE which are bound to the mast cell and cause the release of histamines.
Q: Which Ig is present in foetus?
A: IgG
Q: Which Ig is most abundant in blood?
A: IgG and IgM
Q: Which Ig is present in secretions across the epithelium?
A: dimeric IgA
Q: Which Ig is present in extracellular fluid?
A: IgG
Q: Which Ig coats mast cells below epithelia?
A: IgE
Q: What are IgG good at? Role.
A: binding to virus infected cells and NK cells can recognise cells coated in IgG - Antibody Dependent Cell-mediated Cytotoxicity (ADCC)
