• Antigens

Any substance that specifically stimulates an immune response when introduced into the body is an antigen or immunogen. Several characteristics of antigens are known.

1. Antigens are usually composed of protein or polysaccharide or contain these macromolecules as a major constituent. Lipids and nucleic acids are poor antigens unless linked to a protein or polysaccharide.

2. Some are soluble; others are particulate. Soluble bacterial exotoxins are antigenic. A bacterium may have several types of particulate antigens.

3. In general, larger molecules stimulate a more intense immune response than smaller molecules. Molecules below molecular weight 1000 are very poor antigens.

4. Antigens must be recognized by the host as foreign before they stimulate an immune response.

5. An antigen contains chemically distinct sites, called antigenic determinants (or epitopes), that define its specificity.

6. Small molecules that are nonantigenic by themselves may sometimes become antigenic determinants when coupled to large carrier molecules. These small compounds are called haptens. Haptens react with their specific antibodies, but unless they are attached to larger carrier molecules they are too small to stimulate the production of antibodies.

Thus, the features of antigens that determine immunogenicity in the immune response are as follows:

1. Foreignness: In general, molecules, recognized as «self» are not immunogenic; for immunogenicity, molecules must be recognized as «nonself».

2. Molecular Size: The most potent immunogens are usually large proteins. Generally, molecules with a molecular weight less than 10,000 are weakly immunogenic, and very small ones (e.g., amino acids) are non-immunogenic.

3. Chemical and Structural Complexity: A certain amount of chemical complexity is required. For example, amino acids homopolymers are less immunogenic than heteropolymers containing two or three different amino acids.

4. A ntigenic Determinants (Epitopes): The smallest unit of a complex antigen that is capable of binding to an antibody is known as an antigenic determinant, or epitope. An antigen can have one or more determinants. In general, a determinant is roughly five amino acids or sugars in size.

5. Genetic Constitution of the Host: Two strains of the same species of animal may respond differently to the same antigen because of different composition of immune response genes.

6. Dosage, Route, and Timing of Antigen Administration: Since the degree of the immune response depends on the amount of antigen given, the immune response can be optimized by carefully defining the dosage (including the number of doses), route of administration, and timing of administration (including intervals between doses).

Bacteria may possess one or more antigens: somatic O -antigen (LPS= lipopolysaccharide), capsular K-antigen (polysaccharide or protein), and flagellar H-antigen (protein), exotoxin , etc. (see Fig. 1).1

NOTE: the term “K-antigen” comes from German Kapsule.