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

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 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.

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

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

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.

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

Types of Antigens T-dependent Structure Proteins 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

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.

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 C Similar epitope Cross reactions Anti-A Ab Ag A Anti-A Ab Ag B Shared epitope

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

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 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

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.

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.

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.