Binding of antigen and antibody. Presentation

Binding of antigen and antibidy

1. ANTIGEN ANTIBODY BINDING

2. The interaction between antigen and antibody is called antigen-antibody reaction. Antigen-antibody reaction is the basis of humoral immunity or antibody-mediated immune response. CHARACTERISTICS OF ANTIGEN-ANTIBODY REACTION Antigen-antibody reactions are very specific. Antibodies react with only antigens responsible for their synthesis. The binding takes place between the epitopes of antigens and the paratopes of antibodies. The forces acting between antigen and antibodies are non-covalent forces. The bindings are firm but reversible. ANTIGEN-ANTIBODY REACTION

3. SALIENT FEAUTERES OF ANTIGEN – ANTIBODY REACTION Immune Complex Specificity of Ag- Ab reaction Binding sites of antigen and antibody Binding force of antigen and antibody Avidity Bonus effect Cross reaction

4. STRENGTH OF ANTIGEN-ANTIBODY INTERACTIONS The noncovalent interactions that form the basis of antigen antibody (Ag-Ab ) binding include ; Hydrogen bonds - Hydrogen shared between electronegative atoms (N, O) Ionic bonds- Between oppositely charged groups Hydrophobic interactions -Hydrophobic groups interact unfavorably with water and tend to pack together to exclude water molecules. van der Waals interactions - Fluctuations in electron clouds around molecules polarize neighboring atoms oppositely Electrostatic forces- Attraction between opposite charges Cation -pi interaction- Non-covalent interaction between a cation and an electron cloud of a nearby aromatic group

5. STRENGTH OF ANTIGEN-ANTIBODY INTERACTIONS

6. AFFINITY The strength of binding of one molecule to another at a single site , such as the binding of a monovalent Fab fragment of antibody to a monovalent antigen Affinity refers to the strength of a bond between a particular paratope of antibody and a single antigenic determinant . Antibodies which bind strongly to the antigenic determinant are called high affinity antibodies . Affinity is used to denote the binding capacity of an antibody with a univalent antigen univalent antigen Epitope Antibody Paratope + Antigen –antibody binding due to affinity

7. MULTIVALENT BINDING When complex antigens containing multiple, repeating antigenic determinants are mixed with antibodies containing multiple binding sites, the interaction of an antibody molecule with an antigen molecule at one site will increase the probability of reaction between those two molecules at a second site . A multivalent antigen has many types of antigenic determinanats Multivalent Antigen Elicts the production of different types of antibody The antibodies bound to the respective antigenic determinants

8. AVIDITY The sum total of the strength of binding of two molecules or cells to one another at multiple sites. It is distinct from affinity, which is the strength of binding of one site on a molecule to its ligand. The strength of multiple interactions between a multivalent antibody and antigen is called the avidity . Avidity is used to denote the overall capacity of antibodies to combine with multivalent antigen It is the strength of the bonds after the formation of antigen-antibody complexes Multivalent binding leads to stronger affinity

9. VALENCY Valency refers to the number of antigen-binding sites on an antibody or The number of epitopes (antigenic determinants) on an antigen that can bind to antibodies . Valency determines the antigen-antibody stability Example : IgG has 2 antigen-binding sites → Bivalent IgM ( pentamer ) has 10 binding sites → Decavalent An antigen with 3 identical epitopes → Trivalent Ig G Ig M Antigen with 3 epitopes

10. CROSS REACTIVITY Although Ag- Ab reactions are highly specific, in some cases antibody elicited by one antigen can cross-react with an unrelated antigen. Such cross-reactivity occurs if two different antigens share an identical or very similar epitope. The reaction of an antigen with a closely related antibody is called cross reactivity. The antigen which produce the cross reaction is called cross reactive antigen The cross reaction is due to the presence of one or more identical antigenic determinants on the related antigen. Example: Rheumatic fever-antibodies against Streptococcus bacteria cross react with heart tissue

11. ORIGINAL ANTIGEN CROSS REACTIVE ANTIGEN I dentical D eterminant * * * * * * * * * * * * * * * * * * * * * * * * * Ag- Ab reaction Cross reactivity Antibodies produced against antigen CROSS REACTIVITY

12. THANK YOU Each antigen teaches the antibody something new-just like every challenge teaches us to grow smarter