W2L4 - Immunogens and Antigens

Question 1

Antigen

Answer 1

Any molecule or agent that is able to specifically interact with a component of the immune system
2 types:
Haptens
- any molecule that interacts with the immune but by itself can’t stimulate an immune response
- e.g. urushiol found in sap of poison ivy
Immunogens
- any molecule that can stimulate an immune response

Question 2

Urushiol in Poison Ivy

Answer 2

Sap contains catechol molecules
Catechol molecules must combine with skin proteins before an immune response is elicited
Therefore hapten

Question 3

Hapten Carrier Molecules

Answer 3

A large immunogenic molecule that when a hapten or other non-immunogenic molecule binds becomes immunogenic
E.g. Albumins, Globulins

Question 4

Major Classes of Antigens

Answer 4

Polysaccharides (carbs)
- comprised of long chains of monosaccharides
Proteins
- most proteins immunogenic
- many immune responses due to proteins because of their size and complexity
Lipids
- rarely immunogenic
- can become immunogenic when coupled to protein carrier
Nucleic acids
- poorly immunogenic by themselves
- become immunogenic when coupled to protein carrier

Question 5

Requirements for Immunogenicity

Answer 5

1. Foreignness
   - the more foreign = higher immunogenicity
2. Molecular Size
   - small molecules (less than 1000 Da) = lower immunogenicity
   - large molecules (6000 Da) = higher immunogenicity
   Must have size and complexity to be immunogenic
3. Chemical Complexity
   - increased structural complexity = higher immunogenicity
4. Susceptibility to Antigen Processing and Presentation (Degradability)
   - increased enzyme degradation = higher immunogenicity
   - occurs in APCs

Question 6

Epitopes

Answer 6

The very small part of an antigen that is recognised by the immune system
Present on all antigens
Epitope recognition by:
- Antibodies - immunoglobulins at region end of Y’s
- T cells - T cell receptor
- Surface immunoglobulin on B cells - variable region on IgM and IgD

Question 7

Different Types of Epitope Structure

Answer 7

Conformational Epitope
- aka discontinuous epitope
- relies on 3D structure of molecule and location of target amino acids within structure
- change in shape of whole target molecule may prevent antigen binding
- may result in hidden epitope
Sequential Epitope
- aka linear epitopes
- binding to particular sequence of amino acid
- nucleic acid changes can lead to altered amino acid sequence and inhibit antigen binding

Question 8

Other Requirements for Immunogenicity

Answer 8

Genetic Makeup
Number of Doses
- repeated doses induces stronger IR
Route of Administration

Question 9

Adjuvant

Answer 9

Substance that when mixed with antigen enhances immune response to that antigen

Question 10

Cross Reactivity

Answer 10

Occurs when there is slight modification of an antigen epitope without altering immunogenic nature of molecule
Usually with large molecules with several antigen epitopes
Helps to facilitate vaccination

Question 11

Cross Reactivity and Autoimmune Diseases

Answer 11

Some autoimmune reactions are due to antigenic similarity e.g. rheumatic fever can follow Strep pyogenes infection
Host cardiac muscle has similar antigenic appearance/shape to group A Strep epitope
Region of host antibody molecules were made to react with Strep antigen epitope but antigen binding site also reacts with hosts cardiac epitope

Question 12

What type of antigens do B cells and T cells recognise?

Answer 12

B cells 
- native
- protein, polysaccharide, lipids
- accessible, sequential and non-sequential epitopes
T cells
- processed
- peptides
- internal linear peptides epitopes