Antigens: Structure and Main Properties
Antigens: Structure and Main Properties
Some substances, coming into the body, are able to cause host specific immune response. They were termed “antigens”.
Antigen is any simple or complex substance that elicits specific immune response and interacts with the specific products of immune reactions – antibodies or antigen-specific receptors.
The term antigen is a combination of two primary words “antibody” and “generator”. The term immunogen has the same meaning.
Antigens (Ags) are characterized by the following basic properties: the capacity to trigger the production of immune antibodies or receptors (antigenicity or immunogenicity), and the ability to bind only to the specific antibodies and receptors (antigenic specificity).
Antigenicity depends on various essential properties of an antigen.
Foreignness (or difference from “self”) is the major function of an antigen. In general, molecular structures of the host are recognized as “self and not immunogenic”; for the immune response the molecules must be recognized as “non-self.”
Antigenicity rests on the molecular size, nature and chemical structure of antigens. The potent immunogens are usually large molecular substances. It is commonly assumed that the molecules with a molecular weight less than 10,000 are weakly immunogenic, and very small ones (amino acids, etc.) are non-immunogenic.
Most of antigens are macromolecules such as proteins, polysaccharides, and occasionally lipids or nucleic acids. Molecules differ in their abilty in stimulating antibody production. Proteins and polysaccharides are generally efficient antigens, whereas lipids and nucleic acids are rarely antigenic.
The breakdown of proteins to peptones, amino acids as well as deep denaturation of proteins bring about a loss of antigenic activity, while the introduction of various radicals and side residues into the protein molecule causes the change of antigen specificity.
A certain amount of chemical complexity is required – for instance, amino acid homopolymers are less immunogenic than heteropolymers containing two or three different amino acids.
Genetic constitution of the host substantially affects antigenicity. Two lines of the same animal species respond differently to the same antigen because of another composition of genes, encoding immune response proteins.
Dosage, routs and schedule of antigen administration demonstrate the essential significance for the immune response. Since the degree of immune reactions depends on the amount of antigen given, the immune defense can be changed by different dosage, route of administration, and timing of administration (including the intervals between doses).
The immune response is directed against certain sites of antigenic molecules. Despite the fact that antigens are mainly large substances, the immune response is not directed towards the entire antigen molecule but mainly to the specific chemical groups of its molecule known as antigenic determinants, or epitopes.
Epitope is the smallest unit of a complex antigen that is capable of binding to an antibody. Corresponding site in antibody molecule able to interact with the epitope is named as paratope. Upon large protein molecules, sequences of ten to twenty amino acids act as antigenic determinants. Complex structures such as bacterial cell walls have 100 or more different antigenic determinants.
It is possible to enhance the immunogenicity of an antigen by mixing it with an adjuvant. Adjuvants are the auxiliary substances that boost the immune response – e.g., stimulating the antigen uptake by antigen-presenting cells.
Different antigens (proteins, lipids, polysaccharides and the great variety of small molecules) are characterized by unequal immunogenicity. Thus, there are complete and partial antigens (haptens).
Complete antigens are the substances that elicit full-grade immune response with antibody production solely by themselves (foreign proteins, bacteria and their toxins, viruses, fungal cellular compounds, etc.).
Partial antigens or haptens, if taken solely, do not cause the production of antibodies. They become immunogenic only after binding to a carrier protein. Haptens include large quantity of small molecules (drugs, chemicals, etc), lipids, some carbohydrates, pure nucleic acids and other substances. The addition of proteins to haptens endows them with the properties of complete antigens.