A2 Essential Features of Antigens

Question 1

Intro

Answer 1

• Immunity: the state of resistance to an infection (Erlich)
• Pathogen: a microorganism that causes a disease
• Antigen (Atg): reacts with the products of the specific immune response, antigenicity
• Immunogen Atg: foreign/self-originated molecule/structure which induces immunity
• Hapten: a component with small molecular weight, which is not an immunogenic Atg alone
• Epitop / Atg determinant: part of the Atg that reacts with the product of the specific immune response

Question 2

Characteristics of the Antigen

Answer 2

Antigen: any structure/substance that provokes/induces an adaptive immune response and reacts with the products of the specific immune response, often stimulating the production of antibodies and binding with them.
• Epitope/antigenic determinant: the part of the antigen that reacts with the products of specific immune response.
• Hapten: a small molecule which, when combined with a larger carrier like a protein, can elicit the production of antibodies which bind specifically to it. It is not an immunogenic atg alone.
• Antibody: also known as immunoglobulins, proteins that are produced by the immune system to kill/neutralise pathogens.
• T-cell receptor: molecule that is found on the surface of T-cells or T-lymphocytes that is responsible for recognising antigens.

• Immunogenicity: ability of a molecule to elicite an immune response (ability to induce the production of effector cells or Abs, ie. hapten)
• Tolerogenicity: ability of the Atg to prevent the immune response
• Antigenicity:ability of a molecule to be identified by Atg recognizing immune cells

Question 3

Factors Influencing Immunogenecity

Answer 3

Immunogen: antigens which are capable of inducing adaptive immunity.
• Foreignness (D-amino acid) • Size (number of epitopes) • Chemical composition • Degradability

Epitopes: part of an antigen molecule to which an antibody attaches itself.
• Composition • Structure • Size (4-8 aa) • Number (repeat) • Accessibility

Biological System
• Genotype of the animal • Age

Method of Administration
• Route • Dosage
• Adjuvants • Type of antigen

Question 4

Immunogen

Answer 4

Foreignness
Per recipient organism: how many non-self epitopes are on the immunogen; mostly correlates with size
Size
More epitopes on larger molecules & their distance may be better
Chemical compositions
‒ Proteins: the strongest (tertiary structure, configuration)
‒ Polysaccharide: hardly ever (as glycoprotein)
‒ Nucleic acid: rarely
‒ Lipids: none (have no tertiary structure, poor antigens alone

Question 5

Epitope

Answer 5

• Composition aa sequence: the longer and more complex is better, e. g. Gly polymer is not immunogenic
• Structure ‒ linear (T-cell) ‒ conformation (B-cell)
• Size: 4-8 aa
• Number ‒ many of the same epitopes ‒ many different epitopes
• Accessibility: hidden epitopes

Question 6

Other Factors Determining Antigenicity

Answer 6

Biological factors (individual differences) 
• Genetic factors (MHC, TCR, BCR) • Age (too young, too old) • Condition • Keeping • Feeding

Route of administration
• Parenteral vs. Enteral
• Oral tolerance ↔ oral vaccination

Quantity of antigen • No effect or tolerated
Antigen competition (polyvalent vaccines)
Adjuvants (immunostimulation, depot-effect)
Antigen type (T dependent/independent)

Question 7

Influencing the Effect of the Atg

Answer 7

Booster effect:
the first dose of Atg only generates weak immune response, but a strong immune response is generated by a 2nd exposure to Atg due to immunological memory and the contribution of B & T cells in the acquired immune response.

Adjuvant: Any substance that, when given with an Atg, enhance the immune response to that Atg. It can prolong the presence of the Atg, cause inflammation & activate the innate immune system (complement cascade, phagocytosis)

Question 8

Self / Non-self

Answer 8

1. Visible self Antigens of the body that are tolerated
2. Invisible self Antigens that are not tolerated, but ignored under normal circumstances
3. Visible non-self What the immune system usually reacts with
4. Invisible non-self Non–self antigens that are not reacted with
   The immune system: ‒ attacks foreign (non-self) ‒ tolerates self ‒ considers part of the body as self ‒ the rest of the universe is foreign

Question 9

Types of antigens:

Answer 9

• Self-antigens: antigens which are found on the membranes of almost all cells of the vertebrate host – also known as major histocompatibility antigens. They are glycoproteins in chemical nature.
• Non-self-antigens: any antigen originating from outside the body (e.g. pollen, egg white, protein from other tissues/organs)
• Exogenous antigens: enter the body from the surroundings or environment, by inhalation, ingestion or injection. Should be presented in MHCII.
• Endogenous antigens: are located within the individual itself, generated by the body’s own cells due to faulty proteins synthesis, e.g. infection by bacteria, virus or tumours. Presented in MHCI.
• Autoantigens: these atg should under normal condition not be targeted by the immune system, but due to mainly genetic and environmental factors the normal tolerance have been lost - autoimmune diseases.
• Developmental atg: immune systems cannot recognise them as foreign proteins. They are recognised as self-atg due to the presence of them during foetal life (negative selection by b cells)
o After birth the atg are not produced or present anymore, but some tumour cells can start producing them again.
o Can be detected quite early in the development of the tumour, excellent for diagnostic purposes - good to determine what type of tumour.
o Today also used as targeting cells for AB.

Question 10

Essential features:

Answer 10

• Molecules must be recognised as foreign by the host. • After processing antigens they must undergo some physical and chemical changes that can stimulate the immune system.