Immunogenicity is an substance that induces an immune response and antigen is any substance that is capable of binding specifically to the components of the immune system such as to the antibodies.
Nowadays the terms antigen and immunogens are used interchangeably by immunologists.
Properties of a substance which determine its immunogenicity
Immunogenicity of a substance is determined by following four main properties.
- Foreignness
- Molecular size
- Chemical complexity and
- Stability or degradeability
1). Foreignness
By the term foreign we mean non-self. Our immune system is able to distinguish between self and non-self. To be immuno genic the substance should be genetically foreign to the host. It should be different from the hosts own substances.Immunogen's are mostly peptides or proteins in chemical composition. If the peptide of immunogens have similar nature of the peptides of host organism, there will be no immune response against the antigen.
For example bovine serum albumin (BSA). It is a protein which is found in all cows. So if BSA is injected from one cow into another there will be no immune response, because the protein is identical and for the immune system of every cow.
This BSA is a self molecule but if the BSA is injected into a rabbit an immune response is induced. This is because BSA is foreign to rabbits immune system. So for a molecule to be immunogenic it must be recognised as foreign by the immune system. More foreign and molecule is more is its immunogenicity.
2) Molecular size
Second property which determines the immunogenicity of a substance is its molecular size. It has been found that relatively small substances have decreased immunogenicity whereas large substances have increased immunogenicity.- The most potent immunogens are molecules with molecular weight above 10,000 Dalton's.
- Highly immunogenic molecules have a molecular weight of 1 lakh Dalton's or more.
- Molecules with weight than 10,000 Dalton's are weakly immunogenic or not immunogenic at all.
3). Chemical Composition and Molecular Complexity
Antigens can be carbohydrates, lipids, nucleic acids and proteins chemically but their immunogenicity varies.Carbohydrates are immunogenic only if they have relatively complex polysaccharides structure or when associated with protein carriers. For example blood group antigens.
Lipids and Nucleic acids are not immunogenic by themselves but they become immunogenic when they are conjugated to protein carriers. Proteins are generally the powerful immunogens and this is because of their molecular complexity in size.
The molecular complexity of proteins is a consequence of the variety of units known as amino acids of which proteins are made of. Besides having number of different amino acids proteins have four levels of protein structure.
- Primary structure
- Secondary structure
- Tertiary structure and
- Quaternary structure.
This also contributes to the immunogenicity of a molecule.
Let's take an example, A homo polymer of an amino acid which has a molecular weight of 30,000 Dalton's. Now if we look at its molecular weight it should be immunogenic but this is not the case because of the absence of chemical complexity. Now a copolymer which is composed of different amino acids in its molecular weight is more than 10,000 Dalton's. So it is immunogenic.
Thus molecules with complex nature are more immunogenic when compared to simple molecules.
4). Stability or Degradeability
To stimulate immune response a substance should bind with B-cell receptors or T-cell receptors. B cells interact with the substance on its own that is via its B-cell receptors.But T-cell receptors do not interact with the substance as a whole they interact with a distinct portion of the substance which are short peptides. These small molecules are result of antigen processing and presentation.
Large insoluble macromolecules generally are more immunogenic than small soluble ones. Because larger molecules are more readily degraded and processed and presented to the immune system.
This also explains why proteins are excellent immunogens on the other hand highly stable and non degradable substances are not immunogenic. For example silicon. Thus to be immunogenic a substance must be degradable, highly stable and non degradable substances are not immunogenic.
