Immunology 4: Antibodies Flashcards
What produces antibodies
b lymphocytes
after binding antigen, antibodies initiate what three secondary effector functions
complement activation
opsonisation (promotion of phagocytosis)
cell activation via specific antibody-binding receptors (Fc receptors)
What immunoglobulin catergory are antibodies in
Antibodies are in the GAMMA IMMUNOGLOBULIN category
What is the shape of an immunoglobulin
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
What is the class of protiens that antibodies fall under
immunoglobulins - a large family of soluble glycoproteins
Are all antibodies completely different?
Both light chains and heavy chains can be divided into Variable and Constant regions.
The variable part is where the antigen binding site is so the amino acid sequence must vary to give the antibody different specificities.
The constant part is the Fc part which doesn’t have to be variable because it does not bind to antigens.
Where are disulphide bonds found in the antibodies
Between chains and interchains
The interchains hold together immunoglobulin domains
What are the hypervariable regions
located at loops which are at the end of the protein and interact with antigen.
THREE REGIONS which are HYPERVARIABLE and they are called the Complementarity Determining Regions
Forces involved in antibody-antigen binding
Hydrogen Bonds
Ionic Bonds
Hydrophobic Interactions
van der Waals interactions
What is affinity
The strength of the total non-covalent interactions between a single antigen-binding site and a single epitope on an antigen.
What is avidity
The overall strength of multiple interactions between an antibody with multiple binding sites and a complex antigen with multiple epitopes.
What are antibody cross-reactivity and give two examples
Antibodies produced in response to one antigen can cross-react and bind to a different antigen of similar structure.
Vaccination for cowpox induces antigens which are able to recognise smallpox.
ABO blood group antigens - some antibodies made against microbial agents can cross-react with carbohydrates in erythrocytes e.g. in transfusions, if you give the erythrocytes to someone who has made antibodies against gut bacteria, they can cross-react with receptors on the erythrocytes and destroy them resulting in a severe reaction.
What are the 5 classes of antibodies
Classes
IgG-
gamma heavy chain
3 Ch domains
IgA
alpha heavy chain
3 Ch domains
IgM
Mew heavy chain
4 Ch domains
IgD
Delta heavy chain
3 Ch domains
IgE
epsilon heavy chain
4 Ch domains
Describe the IgG antibody
g heavy chain
Most ABUNDANT immunoglobulin
Monomer
FOUR subclasses: vary mainly in the hinge region and effector function domains
Moves across the placenta
Major activator of the classical complement pathway
There are variations in the hinge region
Some have lots of disulphide bonds and some don’t
Describe the IgA antibody
a heavy chain
Second most abundant immunoglobulin
Blood - Monomer
Secretions - Dimer
Protects mucosal surfaces from bacteria, viruses and protozoa
Secretory IgA
They are joined together by a J chain (joining chain)
Secretory Component - helps protect the molecule from being degraded by the enzymes found on mucosal surfaces
Formation of Secretory IgA
Firstly, the IgA is produced by a plasma cell
The dimeric IgA binds to the Poly-Ig Receptor on the basolateral membrane of the epithelial cell.
This triggers endocytosis
The poly-Ig receptor is cleaved which leaves the secretory component attached to the dimeric IgA
It is then secreted
THE SECRETORY COMPONENT IS DERIVED FROM THE POLY-Ig RECEPTOR
Describe IgM antibodies
m heavy chain
Pentameric
5 monomers joined by the J chain - 10 Fab
Mainly confined to blood (80%)
First Ig produced after exposure to antigen
PRIMARY ANTIBODY RESPONSE
Multiple binding sites compensate for low affinity (high avidity)
Efficient at AGGLUTINATION
Activates complement
Held together by disulphide bonds
IgD
d heavy chain
LOW serum concentrations
Surface IgD expressed early in B cell development
Involved in B cell development and activation
IgE
e 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.
Where are each of the classes distributed
Blood = IgG + IgM
Extracellular Fluid = IgG
Secretions across epithelia (inc. breast milk) = Dimeric IgA
Foetus = IgG
Mast Cells below Epithelia = IgE
What each antibody good at
IgG is good at neutralising
IgM is good at agglutinating
IgG is good at opsonisation
IgM and IgG are good at activating complement
IgE is involved with mast cell sensitisation
IgG is good at binding to virus infected cells and NK cells can recognise cells coated in IgG - Antibody Dependent Cell-mediated Cytotoxicity (ADCC)
