Antigens: Definition, Determinants and Immunopotency

The below mentioned article provides a short note on Antigens:- 1. Definition of Antigens 2. Antigenic Determinants 3. Immunopotency.

Definition of Antigens:

Substances which can give an immune response when introduced into an animal are called immunogens or antigens. Although most antigens are macromolecular proteins, polysaccharides, synthetic polypeptides and other synthetic polymers may also be immunogenic.

Although the characteristics of an­tigens are complex, certain conditions must be sat­isfied in order that a molecule be immunogenic, as follows:

1. The molecules that are foreign to the host are immunogenic.

2. Molecules smaller than MW 10,000 are only weakly immunogenic. Macromo­lecular proteins with molecular weights greater than 100,000 are the most potent immunogens.

3. Immunogenicity increases with structural complexity. Aromatic amino acids are more immunogenic than nonaromatic amino acids.

4. The ability to respond to a particular anti­gen varies with the genetic constitution of the animal.

Antigenic Determinants:

The initiation of immunoglobulin production re­quires binding of the antigen to the lymphocyte surface. The combining sites on the surface of the lymphocytes are antibody-like molecules called antigen receptors.

The portions of antigenic molecules which are involved in actual binding with antibody combin­ing sites are termed antigenic determinants.

Haptens:

1. Karl Landsteiner prepared haptens by covalently coupling diazonium derivatives of aromatic amines to lysine, tyro­sine and histidine residues of immuno­genic proteins. These proteins are called carriers.

2. These are small, chemically defined sub­stances which, although not immuno­genic, react with antibodies.

3. The protein-hapten conjugates form anti-hapten antibody. The conjugated haptens thus behave as the complete antigenic determinant of the molecules.

Size of Antigenic Determinants:

1. Antigenic determinants and antibody combining sites are similar to a lock and key arrangement. The binding affinity be­tween the antigen and antibody site is di­rectly proportionate to the closeness of fit.

2. The antigenic determinant size was per­formed using single sugar (glucose), polysaccharides (dextran). These single- chain polysaccharides with few branch points were used to produce antibodies.

3. The hexasaccharide was the best inhibi­tor of the dextran-anti-dextran precipitant reaction.

Immunopotency of Antigens:

The ability of the region of the antigen molecule to act as an antigenic determinant and to induce the formation of specific antibodies is called immuno-potency.

The following factors influence immuno-potency:

1. Exposure to the aqueous environment is important in immuno-potency.

2. Charge residues contribute significantly to the specificity of antigens, because charged groups are hydrophilic and are in close contact with the environment.

3. Genetic factors play an important role in the ability of different animals to produce antibodies of different specificities against the same antigen. The dominant compo­nent of the antigenic determinant is termed immunodominant.