ANTIGEN

ANTIGEN
Antigen is any substance that stimulates the production of an antibody when introduced into the body.
Antigens are the substances that can be recognized by antibodies or by T cell receptors when complexed with MHC molecules.

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IMMUNOGENS
Immunogens are the substance that can induce the immune response, basically production of B and/or T cells. It is so called immunogenic.

Immunogenicity
It is the ability to induce a humoral and/or cell-mediated immune response.

Antigenicity/immunogenicity
Antigenicity is refered to the properties or features of antigens that determine it's ability to induce the immune response of antibodies
Factors that affect immunogenicity

1. Foreignness: The molecule must be recognised as foreign or nonself. A molecule recognised as 'self' is not antigenic to the individual.

2. Molecular size: The most potent immunogens are proteins with molecular weight above 100,000. Molecules of less than 10,000 molecular weight are weakly immunogenic, e.g. insulin (mol. wt 5808), glucagon (mol. wt. 3800). Small molecules like amino acids are nonimmunogenic.

3. Chemical complexity :
   Most immunogens are proteins. Nucleoproteins are strong immunogens. For instance, Glycoproteins like A and B blood group and Rh antigens are immunogens. Lipoproteins and lipopolysaccharides can be immunogen. Polypeptides like insulin are usually weakly immunogenic. Most lipids are nonimmunogenic. Amino acids are nonimmunogenic.

4. Epitope (Antigenic determinant): Epitope is the specific antigenic site of the antigen molecule. An epitope is the small chemical group on the antigen molecule that elicits and reacts with the antibody or effector T cell. An epitope is a peptide of just 4 or 5 amino acids, or a monosaccharide in size. An antigen molecule may have one or more epitopes.

5. ** Amount of Antigen**: Immune response depends on amount of antigen. The larger the amount of antigen the more antigenic they are.

6. Genetic Makeup of the Host : An example is the alloantigen which is the antigen present in non-identical individuals of the same species, e.g. Blood group antigens.
   Haptens are usually small molecules, e.g. penicillin. Haptens are antigenic but not immunogenic. Haptens are not immunogenic because they cannot activate helper T cells as they are not polypeptides. Hapten also cannot activate B cells by themselves as they are not multivalent macromolecules. Hapten becomes immunogen when combines with a carrier protein. Hapten-carrier protein complex triggers production of anti-hapten antibody with which the hapten reacts specifically.

7. Degradibility: Humoral and cell mediated immunity normally involve the interaction of T cells and antigens where the antigens must be processed and complexed by MHC of antigen presenting cells (APCs).
   Large and insoluble substances are more immunogenic than smaller and soluble because they large substances are more likely to be phagocytosed and processed by APCs. Thus, they are more likely to be expressed at MHC complex.
   The substances that are difficult to be phagocytosed are the poor immunogens.

8. Adjuvants: Adjuvants are substances that, when mixed with an antigen and injected with it, enhance the immunogenicity of that antigen.
   Adjuvants are often used to boost the immune response when an antigen has low or poor immunogenicity or when only small amounts of an antigen are available.

ANTIGENIC DETERMINANT / EPITOPE

Whole antigen molecule does not evoke immune response, only a part of it where antibody can bind & induce immune response. These region regions of the antigen are called epitope.

Epitopes are three types:

1. Linear epitope: Epitopes formed by adjacent amino acid residues.
2. Conformational epitope: Epitopes formed by amino acid but not in sequence become spatially juxtaposed in the folded protein
3. Neoantigenic epitope: New epitopes produced by protein modification such as phosphorylation or proteolysis
   
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TYPES OF ANTIGEN

• Exogenous antigen: These antigens enter the body & circulating in the body fluid and caught by macrophages, dendritric cells etc.

Eg: Bacteria, Virus, fungi etc

• Endogenous antigen: These are the bodies own compounds / fragments/ cell are produced in body are processed by macrophage & later accepted by cytotoxic T cells.

Eg: Histocompatibility Leukocyte antigens (HLA)

• Auto antigen: these are normally proteins that are recognized by immune system of patient suffering from autoimmune disease.

Eg: Nucleoproteins. Nucleic acids etc.

REFERENCE

1. http://www.biologynoteshelp.com/antigenits-property-typeshatenapitope/

2. http://immunologyinfo.weebly.com/antigen.html

3. http://www.ask4biology.com/antigen-and-immunogen/