Lecture 4 - Antibody - Antigen interactions Flashcards
What is the difference between an antigen and an immunogen?
Give examples of each
Immungen: a substance that elicits an immune response
Antigen: target of the immune response
For example:
Immunogen:
• Influenza virion
• Hapten carrier complex
Antigen:
• NA (neuraminadase)
• Hapten
• HA (haemagglutinin)
What is the determinant?
AKA Epitope
The area on the antigen to which the Ab binds
What is Ab?
Antibody, aka immunoglobulin
Molecules made by B cells that bind to antigen
Describe generic variability of Ag.
What about the immunogen?
Antigen can be:
simple: moiety
or complex: protein structure
The immunogen is nearly always complex
What are haptens?
- Small organic moiety found on polypeptides
* target of an Ab response
What is the hapten carrier complex?
- A polypeptide / protein (containing the hapten moiety)
* required for the adaptive immune responsex
Why is the hapten-carrier complex required for the adaptive immune response?
- It is the minimum size required for cross linking on surface B cell receptors
- It contains T cell determinants
Describe binding of the hapten to the carrier molecule
Must be covalently bonded in order to generate an adaptive immune response
Draw a schematic of Ab
Slide 9
What types of molecules can antibodies recognise?
All manner of chemicals:
• protein
• peptide
• haptens
Describe Ab recognition of Ag
Dependent on Complementation of the antigen to the Ag recognition site on the Ab
The recognition site on Ab is very specific for Ag
What is the ‘footprint’
Another name for the Determinant, or Epitope on an Antigen
Describe Complementation in protein Ag
The recognition site on the Ab is specific for the side chains of the determinant.
Single amino acid changes can result in failure of complementation
What types of interactions happen between Ag and Ab?
- Electrostatic bond
- Van der Waals
- Hydrogen bonding
- Hydrophobic force
What are the two types of protein determinants?
Linear
Conformational (discontinuous)
Describe the effect of denaturation on the two types of protein determinants
Linear: no effect
Conformational: conformation is lost, no Ab-Ag binding
Describe some instances of an immunogen having more than one antigen
- Influenza:
• HA
• NA - Hapten-carrier molecule
• anti-hapten antibody
• anti-carrier antibody
• anti-hapten+carrier antibody
What is serum / antiserum?
What does it contain?
Blood depleted of erythrocytes from an immune individual
• contains multiple specificities of antibodies
What does the antiserum of an individual immune to a given strain of influenza contain?
- anti-HA antibodies
* anti-NA antibodies
Describe cross-reactivity
Ab bind to various protein molecules
This comes about because the specific determinant is conserved between various protein molecules
Describe cross-reactivity of flu strains
Influenza virions have many determinants.
Every flu season, the determinants change
From year to year however, there may be cross over of the determinants present on the virion
What is antigenic drift?
Gradual change of viral antigenic determinant through mutation
What is serology?
Use of antibodies to determine structural relatedness of antigens
For example, adding Ab to various strains of virus to determine the presence of the same antigens in the various strains
Compare Antibody and T cell epitopes
T cell epitopes are always linear, whereas epitopes that Ig recognises can be linear or conformational
What is an example of a hapten?
DNP: dinitrophenyl
Describe the response when a single hapten molecule binds to a B cell receptor
This is not a strong enough stimulus to elicit a response
How many B cell receptors need to be engaged in cross linking for a response to be elicited?
Around 20
What is the other name for discontinuous antigen determinants?
Conformational
Are haptens immunogenic?
No
They must be covalently linked to a hapten carrier
What are van der Waals forces?
Weak interaction between atoms
What are hydrogen bonds?
Weak bonds between electropositive and electronegative groups
Draw a happen carrier complex
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