- Antigenicity is the ability to combine specifically with the final products of the responses i.e. secreted antibodies and/or surface receptors on T cells.
- All molecules having the property of immunogenicity also have the property of antigenicity but the reverse is not true.
- Some small molecules are antigenic but they are not able to induce a specific immune response by themselves.
- Such small molecules are called haptens.
- These molecules lack immunogenicity and they can be made immunogenic by addition of large immunogenic protein called a carrier.
- The whole antigen does not evoke immune response.
- There are various small areas of chemical grouping on the antigen molecule which are called antigenic determinants.
- They are also called epitope which determines specific immune response and reacts specifically with antibody.
- Epitopes differ in specificity and potency.
- Each determinant is about 25-34 A in size and 400-1000 M.W.
- The antigenic property is lost by the denaturation of proteins.
- The number of antigenic determinants of an antigen is referred to as its valency which can be of two types i.e. total and functional.
- There are various determinants of antigenicity. They are as follows:
- The immune system has got the capacity of distinguishing between the self and non-self.
- For eliciting an immune response, an antigen must be a foreign substance to the animal (non-self).
- The flip side of the capacity to recognize non-self is tolerance of self, a specific unresponsiveness to self-antigens.
- During the lymphocytes development, much of the ability to tolerate the self-antigens arises.
- This occurs only when immature lymphocytes are exposed to self-components.
- Cells are inactivated that are responsible in recognizing self-components during this process.
- Antigens may be recognized as non-self, or foreign during the critical period by the immune system which have not been presented to immature lymphocytes.
- The degree of immunogenicity determines the degree of foreignness.
- The phylogenetic distance between two species if is greater, then the structural disparity between their constituent molecules will also be greater.
- For example: Bovine serum albumin (BSA) is strongly immunogenic to rabbit but not in cow which is a common experimental antigen.
- Similarly, BSA would be expected to show greater immunogenicity in a chicken than in goat, which is more closely related to bovines.
- But, there are some exceptions to this rule as well.
- Some macromolecules are present like collagen and cytochrome which have been highly conserved throughout the evolution.
- For this reason, they express very little immunogenicity across diverse species line.
- There are some self-components like corneal tissue and sperm which are effectively sequestered from the immune system.
- But if these tissues are injected into the animal from which they originated, they will behave as an immunogens.
- Molecular size is found to be closely related to the antigenicity.
- Large molecules with high molecular size of 6.75 million Daltons like haemocyanin and other products like tetanus toxoid, egg albumin, thyroglobulin having molecular size of 14000-600000 Daltons are highly antigenic.
- Substances having less molecular weight (insulin) than 10000 daltons are either non antigenic or weakly antigenic.
- Proteins and some polysaccharides are mainly naturally occurring antigens.
- Proteins are more effective than polysaccharides in comparison to stimulate antibody production.
- There are some exception for gelatin, histones and protamines which are non-antigenic due to their low tyrosine content (aromatic radical).
- Presence of aromatic radical is essential for rigidity and antigenicity of a substance.
D)Susceptibility to tissue enzymes
- Tissue enzymes can convert some substances to soluble forms during metabolism which can act as good antigens.
- Antigens introduced into the body are degraded by the digestive enzymes of the phagocytic cells.
- Then they are broken down into smaller fragments of appropriate size which contains the antigenic determinants.
- Substances that cannot be metabolized and converted to soluble forms are not antigens.
- Only some portions of the antigen show antigenic specificity with some specific active sites whereas the remaining portions are antigenically inert.
- The determinant group is made up of six sugar residues that can act as antigenic determinant for polysaccharides molecules.
- For protein molecules, it requires 5-7 amino acids residues and five nucleotides are required for nucleic acid.
- The antigens are multivalent and large antigens possess hundreds of different determinant groups.
- Species specific antigens are present in tissues of all individuals in a particular species.
- Thus, human blood proteins can be differentiated from the animal blood proteins by specific antigen-antibody reaction.
- Alloantigens or isoantigens are found to be present in some members in a species but not in all the members.
- They are able to produce isoantibodies or alloantibodies.
- Human blood is divided into various blood groups on the basis of human erythrocyte antigens and Rh antigens.
- Histocompatibility antigens are present in the plasma membranes of tissue cells.
- Organs specific antigens are confined to a particular organ or tissue.
- Certain proteins of brain, kidney, thyroglobulin and lens protein of one species share specificity with that of another species.
- The autologous or self-antigens are ordinarily not immunogenic.
- But under certain circumstances lens protein, thyroglobulin and others may act as autoantigens.