What is Antigen-Antigenicity-Types of antigens-Properties of antigenicity .

ANTIGEN :

An antigen is a substance which, when introduced into a body evokes immune response to produce a specific antibody with which it reacts in an observable manner.

Antigenicity :

Ability of an antigen to stimulate the immune response in the host that means capacity to elicit the production of antibody. Antigens that are capable of inducing an immune responses are called immunogens.

Ability of an antigen to trigger the immune response and to be bound over the product resulted from both humoral immunity and cell mediated immunity is called antigenicity .

Types of antigens

Complete and incomplete antigens:

a.       Exogenous antigens :

·         Antigen arises from the outside environment of the body to induce the production of an antibody is known as exogenous antigen. (e.g. Bacteria, viruses and fungi).

·         Usually these exogenous antigens are taken up by APCs and expressed along with MHC-II molecule , which in turn activates T cell there by causing cellular destruction by cytokines. E.g. bacteria,viruses, fungi etc.

·         Some antigens might be exogenous at the initial stage, but later it become endogenous.

b.      Endogenous antigens:

·         Antigens or antigenic product of our own body’s cells or antigenic product the endogenous antigens are processed by the macrophages which are later accepted by the cytotoxic T- cells.

·         Endogenous antigens include xenogenic , autologous and idiotypic or allogenic antigens. E.g. Blood groups antigens, HLA etc.

c.       Autoantigens :

·         Auto antigen is usually a normal protein or complex of proteins that is recognized by the immune system of patients who has been suffering from a specific auto immune disease. E.g. Nucleoproteins, Nucleic acids,etc.

1.      Complete antigen :

These are substance which can induce antibody formation by themselves and can react specifically with these antibodies.

2.      Haptens antigen:

Haptens are substances which are unable to induce antibody formation on its own but can become immunogenic (capable of inducing antibodies) when covalently linked to proteins called carrier proteins.

Antigenic determinant (editope) is the smallest unit of antigenicity.

Antigen possesses a number of these determinants.

Each type of determinant induces a specific antibody formation.

Properties  of antigenicity :

The exact basis of antigenicity is not clear but a number of factors have been implicated which make a substance antigenic.

These factors are as follows:

1.      Foreignness :

An antigen must be foreign to the individual (non self) to induce an immune response.

2.      Size :

Larger molecules are highly antigenic whereas low molecular weight is either non-antigenic or weak antigenic.

3.      Chemical nature :

·         Antigens are either protein or polysaccharides.

·         Proteins are more effective antigens.

·         The vast majority of immunogens are proteins. These may be either pure protein or glycoproteins or lipoproteins.

·         Polysaccharides : pure polysaccharides and lipopolysaccharides are the good immunogens.

·         Nucleic acids: nucleic acids are poor immunogenic. However, they may become immunogenic when single stranded or complexed with proteins.

·         Lipids: in general lipids are non-immunogenic , although they may be haptens.

4.      Physical form :

In general particulate antigens are more immunogenic than soluble ones. Denatured antigens are more immunogenic than the native form.

5.      Species Specificity:

Tissues of all individuals in a species possess species specific antigens.

6.      Isospecificity :

A species may be grouped according to the presence of different isoantigens in its members. Blood grouping is one example depending on human erythrocyte antigens (isoantigens).

7.      Auto specificity :

Self antigens are generally non-antigenic but there are exceptions.

8.      Organ specifity :

Organ specific antigens are confined to a particular organ. Some organs like brain, kidney,and lens protein of one species share specificity with that of another species.

9.      Heterophile specificity :

The same or closely related antigens present in different tissues of more than one species are known as heterophile antigens. Antibodies to these antigens produced by one species cross react with antigens of other species.