1. Antigen and Antigenicity Antigen and Antigenicity
Nada Mohamed Ahmed ,
MD, MT (ASCP)i

2. Objectives Terms definition Characteristic of good immunogen
Antigenic determinants
Types of Ags according to types of cells recognized Antigenic determinants .
interaction between antigen and antibody

3. Terms definition 1 Antigen
Any molecule that can be specifically recognized by the adaptive elements of the immune system, that is by B cells or T cells, or both .
2 Immunogenicity
Immunogenicity is the ability to induce a humoral and/or cell mediated immune response.
3 Antigenicity (Reactogencity)
Antigenicity is the ability to combine specifically with the final products of the above responses (i.e.,antibodies and/or cell-surface receptors).
4 Complete antigen, Immunogen
Substance own both ability above.

4. Characteristics of a good immunogen?
1. Foreignness
2. Molecular size:
Excellent: >100,000 da
Fair-poor: 5,000-10,000 da
3. Chemical Composition & Heterogeneity.
4. For TCR: ability to be processed & displayed in the MHC.
02/01/08

5. Chemical nature of immunogens:
Proteins: Excellent immunogens.
Polysaccharides: Good
LPS
DNA: poor
Denature- hapten
Lipids: poor
hapten

6. Epitope (antigenic determinant)
Epitopes are the immunological active regions of an immunogen, which bind to antigen-specific membrane receptors on lymphocytes or to secreted antibodies
The portion of antigen molecules which
can be specifically bound by antibody or
antigenic receptor of lymphocytes.

7. Types of Antigens T-independent
Types of Ags according to types of cells recognized Antigenic determinants .
Types of Antigens T-independent
Structure
Polysaccharides
Properties
Polymeric structure
Polyclonal B cell activation
Resistance to degradation
Examples
Pneumococcal polysaccharide, lipopolysaccharide
Flagella

8. Types of Antigens T-dependent
Structure
Proteins
Examples
Microbial proteins
Non-self or Altered-self proteins

9. Interaction between antigen and antibody
1 key –to-lock- like spatial configuration
2 non-covalent bond: hydrogen bonds, electrostatic interacton, van der waals forces and hydrophobic force
3 reversible
the interaction can be impacted by temperature, isoionic point, ionic strength and ratio of antigen to antibody.
4 Affinity of antibody: Antibody affinity is the strength of the reaction between a single antigenic determinant and a single combining site on the antibody.
5 cross reaction
The ability of a population of Ab molecules to react with more than one Ag

10. How AG induce Immune Response
Antibodies are produced by plasma cells, but, once secreted, can act independently against extracellular pathogen and toxins.
Antibodies bind to specific antigens on pathogens; this binding can inhibit pathogen infectivity by blocking key extracellular sites, such as receptors involved in host cell entry.
Antibodies can also induce the innate immune response to destroy a pathogen, by activating phagocytes such as macrophages or neutrophils, which are attracted to antibody-bound cells.

11. Affinity describes how strongly a single antibody binds a given antigen, while avidity describes the binding of a multimeric antibody to multiple antigens.

12. Cross Reactivity
The ability of an individual Ab combining site to react with more than one antigenic determinant.
The ability of a population of Ab molecules to react with more than one Ag
Anti-A Ab
Ag C
Similar epitope
Cross reactions
Anti-A Ab
Ag A
Anti-A Ab
Ag B
Shared epitope

13. Epitope and idiotope Antigen determinant antigen epitope
Antigen-binding site
Constant domain
variable domain
L-V region
H-V region
antigen
antibody

14. Antigenic Determinants Recognized by B cells and Ab
Composition
Proteins, polysaccharides, nucleic acids, haptens
Sequence (linear) determinants
Conformational determinants
Size
4-8 residues

15. Antigenic Determinants Recognized by B cells and Ab
Composition
Size
Number
Limited (immunodominant epitopes)
Located on the external surfaces of the Ag Fe

16. Antigenic Determinants Recognized by T cells
Composition
Proteins (some lipids)
Sequence determinants
Processed
MHC presentation
Size
8 -15 residues
Number
Limited to those that can bind to MHC

17. Antigen and Antibody Reactions
Type of reaction:
Sensitization: simple combination between red cell antigen and corresponding antibody in a vivo or vitro without visible agglutination.
Hemoagglutiation: reaction between red cell bound antigen and corresponding antibody with visual agglutination.

18. Hemolysis: reaction between red cell bound antigen and antibody with complement fixation resulting in releases of hemoglobin.
Neutralization (inhibition): reaction between red cell antigen and corresponding antibody both in soluble state with precipitation.

19. Factors affect Antigen Antibody Reaction
Complete or incomplete, cold or warm, natural or immune types of antibodies.
Amount of antibody.
Amount of antigen.
Avidity of antibody.
PH of the reaction.
Temperature of the reaction.
Incubation time.
Saline concentration.
Type of additive (protein, enzyme).
Speed and time of centrifugation.