I- INTRODUCTION, DEFINITIONS :
Called antigen any natural or synthetic molecular species capable of & rsquo; inducing an immune response in a living organism and react specifically with the products of this response (BCR, antibody, TCR).
An antigen is a substance which is :
– Induce an immune response.
It is said that & rsquo; antigen has a d & rsquo property; immunogenicity.
– Be recognized by an antibody or lymphocyte (T or B). It is said that & rsquo; antigen has a d & rsquo property; antigenicity.
L & rsquo; & rsquo induction; an immune response by inoculation & rsquo; an immunogen in a living organism.
Epitope or epitope :
In an antigen, only specific sites are responsible for antigen reactivity. These sites are called antigenic epitope or determinant.
An antigen typically has several epitopes, often different.
Each epitope has a complementary three-dimensional structure of the binding site of the molecule & rsquo; antibody (the recognition site or paratope).
Chemical substance of low molecular weight which has an antigen reactivity but n & rsquo; is not immunogenic. C & rsquo; is equivalent to an isolated antigenic determinant. It can become immunogenic if the couple to a large carrier molecule.
Therefore, All immunogens are antigens, some antigens are not immunogenic.
II- CLASSIFICATION OF ANTIGENS :
A- According to the & rsquo; origin :
1- Natural antigens :
- Autoantigens : they belong to a given individual. They are able to & rsquo; induce the production of & rsquo; antibody within the & rsquo; organism they come from ; they are called auto-antibodies and are responsible for autoimmune diseases.
- The alloantigens : Antigens that characterize groups of & rsquo; genetically different individuals within d & rsquo; a species (Example : allo-antigens ABO blood).
- xeno-antigen or the hetero-antigen : worn by agencies & rsquo; a different species.
2- Artificial antigens :
They are obtained after modification of the natural antigens.
3- Synthetic antigens
B- Depending on the structure :
particulate antigens :
What are the biggest antigens. It s & rsquo; these microorganisms, erythrocyte, latex particles…etc.
soluble antigens :
These are molecules. It s & rsquo; is protein, polysaccharides…etc.
C- Depending on the nature of immune responses produced :
Depending on whether or not the & rsquo; T cell help for the production of & rsquo; antibodies are distinguished :
1- Thymus-dependent antigens :
Involve the participation of T cells in the production of & rsquo; antibodies.
The humoral immune response to thymus-dependent antigens is IgG, high affinity and with memory cells.
2- T-independent antigens :
N & rsquo; implies that the participation of B cells.
The humoral immune response to T-independent antigens type IgM, low affinity and cell-free memory.
III- CONDITIONS OF & rsquo; Immunogenicity :
A- Features related to the & rsquo; antigen :
1- foreign character of & rsquo; antigen :
Also called phylogenetic distance, it corresponds to the degree of’ “strangeness” de l’antigène.
Generally, a body n & rsquo; not induce an immune response against its own constituents ; More foreign substance is compared to itself, over its immunogenicity is high.
2- chemical nature :
Inorganic compounds do not stimulate lymphocytes.
For organic compounds :
- Proteins are the most potent immunogens ; Proteins are very antigenic molecules due to the polymorphism of their structure and differences between species and between individuals within the same species.
- Polysaccharides and the polysaccharides are poorly immunogenic.
- The lipids themselves are not immunogenic, because their structure is substantially the same in many animal species : that are haptens.
- L & rsquo; pure DNA and isolated n & rsquo; is not immunogenic.
3- molecular weight and size :
The higher the molecular weight is high, more powerful is its immunogenicity. And just, plus size d & rsquo; a molecule is large, more normally immunogenicity is high.
4- chemical complexity :
Over a molecule is composed & rsquo; different elements, it is more immunogenic.
5- catabolism :
More catabolism is slow, more antigenic stimulation persists and increases immunogenicity.
B- Terms & rsquo; s Administration & rsquo; antigen :
1- Route of & rsquo; introduction :
The route of & rsquo; introduction of the & rsquo; antigen has a strong influence on & rsquo; s body where the cell population of the immune system that will be involved in the response.
The most effective is one that allows the fastest and most prolonged touch between & rsquo; antigen and the cells of the & rsquo; immunity.
The ways d & rsquo; most effective immunization are intramuscular, the subcutaneous and intradermal.
2- Dose d & rsquo; antigen used :
- If & rsquo; antigen is introduced in quantities too small, it n & rsquo; there is no reaction.
- From d & rsquo; a certain threshold, the reaction is proportional to the amount administered.
- If & rsquo; antigen is excessive amount, the response decreases up up & rsquo; to immune tolerance.
Usually, a single dose of most Ag n & rsquo; not induce a strong response, in fact, a repeated administration over a period of several weeks is necessary to induce a strong immune response (principle of vaccine reminders).
3- Using & rsquo; admixtures :
Adjuvants administered simultaneously with the & rsquo; antigen, are components that increase & rsquo; immunogenicity of & rsquo; antigen.
They are frequently used to unleash the immune response d & rsquo; a weakly immunogenic antigen, or when it is available only in small quantities.
They work by forming deposits from the & rsquo; antigen is gradually released. They thus stimulate a local inflammatory response which attracts phagocytes and lymphocytes. So, contact of the & rsquo; antigen with competent cells is extended.
C- Factors related to the & rsquo; host :
1- The genotype of the recipient :
The genetic makeup of & rsquo; an immunized organism, affects the type of immune response to it, as well as the degree of this response. These genes are, largely, Major genes Complex d & rsquo; Histocompatibility (CMH).
So, we distinguish among subjects stimulated by a given antigen, good responders and poor responders.
2- L & rsquo; age :
It affects the & rsquo; physiological state of development of the immune system and the quality of the immune response (the type of the response, l & rsquo; intensity of the response…).
IV- MOLECULAR BASIS L & rsquo; ANTIGENICITY :
* L & rsquo; immunogen should have a minimum size, have a certain molecular complexity, have multiple determinants of size d & rsquo; about 8 at 20 amino acids and must be accessible.
* An antigen has a number of antigenic determinants underlying the specificity.
* The recognition of & rsquo; an antigen by an antibody or a receptor for & rsquo; antigen (BCR, TCR) is characterized by ” the specificity “, However, there are reactions in which said crossed ;
- An antibody recognizes two or more different antigens (different antigens but expressing an epitope common).
- An antibody recognizes two or more different epitopes but very similar structure.
- An epitope can be recognized by two or several different antibodies.
Course of Dr H. BOUAB – Faculty of Constantine