1. Antigen – Antibody Reactions
An antigen is a substance/molecule that when introduced into the body triggers the production of an antibody by the immune system which will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as pollen or cells such as bacteria.

2. Properties of Antigen-Antibody Interaction:
1- Lock and Key concept :
The antigenic determinant or epitope is recognized by antibody binding site, one of them as a key and the other as a lock.

4. 2- Non – covalent Bonds:
The Ag-Ab interaction is due to lots of non-covalent bonds include :
Hydrophobic bonds.
Hydrogen bonds.
Electrostatic bonds.
Van der Waal interactions.
Ionic bonds.

5. 3- Reversible:
Reversible reaction because the bonds are non-covalent and weak.

6. Factors Affect Detection of Ag-Ab Reactions :
1- Affinity :
The strength of interaction between an epitope and an antibody’s antigen binding site.
High affinity between Ag-Ab is easier to be detected.

7. 3- Avidity :
Avidity gives a measure of the overall strength of an antibody-antigen complex.
High Avidity is easier to be detect.

8. 4- Ag-Ab ratio:
Concentration of Antigen and Antibody Affect the binding strength of the product complex.
5- Physical form of the antigen :
Liquid antigen is different from solid ,must choose the right method to detect the interaction,e.g. detection of liquid by precipitation

9. Primary and secondary antibody:
Antigen
Injection in foreign animal
Primary antibody
Injection in foreign animal
Secondary antibody

10. Tests of Ag-Ab detection :
1- Agglutination Reaction :
Detection of Antigen in a liquid sample like blood , in this reaction the Ag-Ab binding cause agglutination called Hemagglutination in case of RBCs.
Types of Agglutination tests :
A- Direct agglutination.
B- Indirect agglutination.
C-passive agglutination.

11. A- Direct agglutination :

12. B- Indirect agglutination.
To test patient serum for the presence of antibodies against soluble antigens, serum is mixed with latex spheres with the soluble antigens attached.
Antibodies will then cause visible agglutination of the latex spheres with the soluble antigens attached.
Alternatively, antibodies may be attached to the latex spheres to test for the presence of soluble antigens in patient serum.

14. C-passive agglutination.

15. Agglutination tests are used in :
Blood type tests .
Bacteria infection diagnosis.

16. 2- Radioimmunoassay :
RIA uses radiolabeled antibodies. The presence of target proteins or antibodies is determined by measuring the amount go radioactivity.

17. 3- ELISA :
Enzyme-linked immunosorbent assay (ELISA), also known as an enzyme immunoassay (EIA), is a biochemical technique used mainly in immunology to detect the presence of an antibody or an antigen in a sample.
ELISAs are quick and simple to carry out, and since they are designed to rapidly handle a large numbers of samples in parallel

18. Types of ELISA :