Nada Mohamed Ahmed ,MD, MT (ASCP)i

Antigen and Antigenicity

Antigen and Antigenicity

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

Terms definition

1 AntigenAny 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 ImmunogenicityImmunogenicity 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.

02/01/08 4

• 3. Chemical Composition & Heterogeneity.

• 4. For TCR: ability to be processed & displayed in the MHC.

Characteristics of a good immunogen?

• 1. Foreignness

• 2. Molecular size:– Excellent: >100,000

da

– Fair-poor: 5,000-10,000 da

5

Chemical nature of immunogens:

• Proteins: Excellent immunogens.

• Polysaccharides: Good– LPS

• DNA: poor– Denature- hapten

• Lipids: poor– hapten

Epitope (antigenic determinant)

The portion of antigen molecules which can be specifically bound by antibody or antigenic receptor of lymphocytes.

Epitopes are the immunological active regions of an immunogen, which bind to antigen-specific membrane receptors on lymphocytes or to secreted antibodies

Types of AntigensT-independent

• Properties– Polymeric structure– Polyclonal B cell

activation– Resistance to

degradation

• Examples– Pneumococcal polysaccharide, lipopolysaccharide– Flagella

Types of Ags according to types of cells recognized Antigenic determinants .

• Structure • Polysaccharides

Types of AntigensT-dependent

• Proteins

• Structure

• Examples– Microbial proteins– Non-self or

Altered-self proteins

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

•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.

How AG induce Immune Response

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

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 A

Anti-A Ab

Ag B

Shared epitope

Anti-A Ab

Ag C

Similar epitope

Cross reactions

antibody

Epitope and idiotope

epitopeAntigen determinant

idiotope

Antigen-binding site

Constant domain

variable domain

L-V region

H-V region

antigen

Antigenic Determinants Recognized by B cells and Ab

• Composition– Proteins, polysaccharides, nucleic

acids, haptens– Sequence (linear) determinants– Conformational determinants

• Size– 4-8 residues

Antigenic Determinants Recognized by B cells and Ab

• Composition• Size• Number– Limited

(immunodominant epitopes)

– Located on the external surfaces of the Ag

Fe

Antigenic DeterminantsRecognized 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

Antigen and Antibody Reactions

Type of reaction:I. Sensitization: simple combination

between red cell antigen and corresponding antibody in a vivo or vitro without visible agglutination.

II.Hemoagglutiation: reaction between red cell bound antigen and corresponding antibody with visual agglutination.

II.Hemolysis: reaction between red cell bound antigen and antibody with complement fixation resulting in releases of hemoglobin.

IV.Neutralization (inhibition): reaction between red cell antigen and corresponding antibody both in soluble state with precipitation.

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.