Antigenic determinants and epitopes
The part of the antibody molecule which makes contact with the
antigen is termed the paratope. Consequently, the part of the antigen
molecule that makes contact with the paratope is called the epitope. As
most antigens are protein in nature, they exist in a folder, three dimensional,
tertiary structure. Hence there may be a cluster of amino acid sequences on
the three dimensional structure constituting a series of epitopes. Each of these
epitope clusters on the antigen are known as an antigenic determinant.
Generally the substances, which are capable of eliciting an immune
response, that is, the synthesis of antibodies are called antigens. However
the above definition of antigens do not infer the cell mediated response. Hence
the term immunogen is introduced to denote antigenic substances which elicit
both humoral and cell mediated immune response or either humoral or cell
mediated response. (Antigenic determinants and epitopes)
Antigens may be either natural or artificial/synthetic ones. Natural
antigens, fall into two categories viz., particulate antigens and soluble
antigens. Bacteria, viruses, erythrocytes and cells represent particulate type
antigens. Bacterial toxins, proteins, carbohydrates, glycoproteins and
lipoproteins represent the soluble antigens.
In addition, a variety of chemical compounds, biological
macromolecules, synthetic polypeptides are regarded as potential antigens.
Several polysaccharides, nucleoproteins and lipoproteins are also considered
as antigens. Recently antibodies to DNA have been used in immunization. (Antigenic determinants and epitopes)
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