1. Antigen

2. An antigen is a substance which when introduced in the body evokes immune response to produce a specific antibody with which it reacts in an observable manner.

3. Types of Antigen
Complete Antigen: Substances which can induce antibody formation by themselves.
Haptens: substances which are unable to induce antibody formation on its own but can become immunogenic, i.e., capable of inducing antibodies, when covalently linked to proteins known as carrier proteins.

4. Types of Haptens
2. Simple Haptens: combine with specific antibodies but no precipitate is produced.
1. Carrier haptens: combine with specific antibodies to form precipitates.

5. Factors of Antigenicity
Foreignness
Size
Chemical nature
Susceptibility to tissue enzymes
Antigenic specificity
Species specificity
Isospecificty
Autospecificity
Organ specificity
Heterophile specificity

6. Foreignness
An antigen must be foreign to the individual to induce an immune response.
Immune system does not normally mount an immune response against his own antigens(self).
The more foreign a substance, it is likely to be more powerful antigen.

7. Size
Larger molecules are highly antigenic whereas low molecular weight (less than 10000) are either non antigenic or weakly antigenic.

8. Chemical nature
Most naturally occurring antigens are either proteins or polysaccharides
Proteins are more effective antigens.
Exception is gelatin: a non antigenic protein due to low tyrosine content
Lipids and nucleic acids are less antigenic on their own but do so when combined with proteins

9. Susceptibility to Tissue Enzymes
Substances which can be metabolized and are susceptible to the action of enzymes behave as antigens
The hidden antigenic determinants can be identified only when the molecule is broken down by enzymes.
Substances unsusceptible to tissue enzymes are not antigenic.
Substances which cannot be metabolized in the body are not antigenic.

10. Antigenic Specificity
It depends on antigenic determinants.
It is not absolute. Cross reactions may occur between antigens which bear stereochemical similarities.

11. Species specificity
Tissues of all individuals in a species specific antigens.
It has got phylogenetic relationship and helps in tracing evolutionary relationship and forensic applications in identification of species of blood and of seminal fluid.

12. Isospecificity
Depends on isoantigens found in some but not all members of a species.
These are genetically determined.
Blood grouping is one such example depending on human erythrocyte antigens (isoantigens)
Histocompatibility antigens are associated with plasma membrane of tissue cells.

13. Autospecificity
Self antigens are generally non antigenic, but there are exceptions like lens proteins and sperm
When these antigens are released into the tissues following injury, antibodies are produced against them
Antigenic specificity of self antigens may be modified as a result of infection or irradiation and thus may act as non self.

14. Organ specificity
Organ specific antigens are confined to a particular organ.
Some organs like brain, kidney and lens protein of one species share specificity with that of another species.

15. Heterophile specificity
Same or closely related antigens present in different tissues of more than one species are known as heterophile antigens.
Antibodies to these antigens produced by one species cross react with antigens of other species.