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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.