Antigen-antibody reaction

1. Antigen-antibody reaction Immunology 2011

2. What is antigen-antibody reaction? • The interaction between antigen and antibody • ag - ab reaction • Antigen-antibody reaction is characterized by following salient features. • Immune Complex • Specificity of Ag-Ab Reaction • Binding Sties of Antigen and Antibody • Binding Forces of Antigen and Antibody • Avidity • BonusEffect • Cross Reaction

3. Immune Complex • When antigen and antibody are brought together, the antibody binds with the antigen to form a complex molecule called immune complex or antigen- antibody complex • Ag + Ab Ag-Ab complex

4. Specificity of Ag-Ab Reaction • Specificity refers to the discriminate ability of particular antibody to combine with only one type of antigen. • An antibody will combine with the antigen which is the cause for its production.

5. Binding Sties of Antigen and Antibody • Epitope (antigenic determinant)- The part of the antigen which combines with the antibody. • Paratope (antigen binding site) - The Part of the antibody which combines with the antigen.

6. Binding Forces of Antigen and Antibody • Binding between antigen and antibody is due to three factors namely, • Closeness between antigen and antibody • Intermolecular forces • Affinity of antibody

7. Avidity • Avidity refers to the capacity of an antiserum containing various antibodies to combine with the whole antigen that stimulated the production of antibodies. • Avidity is used to denote the overall capacity of antibodies to combine with multivalent antigen

8. Bonus Effect • the antibody not only binds with the antigen but also the antigens are bridged by a single antibody. • In some cases two antigens may be bridged by a single antibody. • Such a binding is weak. • But when two antigens are bridge by two antibodies, the binding will be strong. • This phenomenon of giving extra- strength to the antigen-antibody complex by the binding of two antibodies to two antigen molecules is called bonus effect.

9. Cross Reaction • An antiserum raised against a given antigen may sometimes react with another closely related antigen - cross reaction • the antigen which produces 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.

10. Types of Antigen-antibody Reaction) • Precipitation • Agglutination • Cytolysis • Complement fixation • Flocculation • Opsonization • Immunoflurorescence

11. Precipitin Reaction) • reaction between a soluble antigen and its antibody resulting in the formation of insoluble precipitate. • The antibody - precipitin. • due to the formation of Antigen-antibody complex. • The antigen is multivalent and the antibody is bivalent.

12. As each antibody is a bivalent molecule, it can bridge two multivalent antigen molecule. • This bridging leads to the formation of a lattice which forms the precipitate. • When antigen and antibody are in optimal concentration the precipitation is complete and a large lattice is formed.

13. Agglutination • reaction where the antibody of serum causes the cellular antigens to adhere to one another to form clumps. • Antibodies - agglutinins • antigens – agglutinogens • When red blood cells are agglutinated, the reaction - haemagglutination. • bacterial agglutinations

14. Cytolysis • is the dissolution of a cell. • RBC is lysed - haemolysis. • bacterial cell - bacteriolysis. Mechanism of Cytolysis: • the antigen-antibody complex activates the complement. • complement binds to the surface antigen of microbe or cell. • The compliment fixed on the surface of the cell causes the disruption of the lipid bilayer of the membrane of the microbe. • As a result, a hole is made on the microbe. • Through this hole the content of the cell are released and the cell is lysed.

15. Complement Fixation • The binding of complement to antigen –antibody complex is called complement fixation. • When complement is added to a serum containing an antigen and its antibody, the complement is activated and • immediately it binds to the Antigen-antibody complex and the complement is said to be fixed.

16. Flocculation • antigen- antibody reaction brought about by exotoxin and antitoxin. • reaction produces flocculates which do not sediment but remain dispersed in the medium. • Flocculation is somewhat like precipitation, but the precipitates will not sediment.

17. Opsonization • Process by which a particular antigen becomes more susceptible to phagocytosis by combination with an opsonin. • Opsonin is an antibody • when combines with a particulate antigen, increases the susceptibility of the antigen to phagocytosis • In Opsonization - the antibody combines with the surface antigen of bacteria.

18. This antigen-antibody complex activates complement system. • The activated complement is attached to the antigen-antibody complex to form an antigen-antibody complement complex. • the antigen-antibody-complement is adhered to the phagocytic cells. • The microbe ( antigen) fixed on the phagocytic cell is killed by phagocytosis or lysis.

19. Immunofluorescence • When antibodies are mixed with fluorescent dyes such as fluorescein or rhodamine, they emit radiation. • Phenomenon of emitting radiation by antibodies labeled with fluorescent dye- immunofluorescence. • The immunofluorescence can be observed by a fluorescent microscope. • Application of Immunofluorescence • It is used to locate and identify antigen in tissues. • Certain pathogenic bacteria can be identified. • Antibodies directed to cell or tissue antigen can be detected.

20. Method of Immunofluorescence Direct method • the antibody labeled with fluorescent dye is applied directly on the tissue section. • The labeled antibody binds with specific antigen. • This can be observed under the fluorescent microscopic. • rabies can be diagnosed.