Antigens

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I- INTRODUCTION, DEFINITIONS :

Antigen :

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.

Immunisation :

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).

hapten :

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 :

A = antigen more epitopes

* 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.

B epitope / T epitope – linear epitope (sequential) / conformational epitope

* 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