Classification of Antigen
1. According to immunogenicity (ability to induce antibody formation).
Antigen (Ag's) are;
1. Complete antigen
High molecular weight proteins.
It is able to produce antibody formation and produce a specific/ observable reaction with that antibody.
2. Haptenes
Those substance not capable of inducing antibody formation but react specifically with antibodies.
When heptens combine with larger molecule carrier then only capable of inducing antibodies.
3. Epitope (antigenic determinant)
It is the smallest unit of antigenicity.
Specific reaction with antibodies.
Structure of an Ag Molecules
a. Antigenic determinant site
b. Ag- binding site
c. Antibody
2. Depending on antigen ability to induce antibody formation.
Antigens are of two types;
1. T- cell dependent (TD) antigen
2. T-cell independent (TI) antigens
1. T- cell dependent (TD) antigen
These antigens stimulate antibody production. Antibodies are produced by B-cells with the help of Tcells, this known as T-cell dependent antigens
2. T-cell independent (TI) antigens
Some antigens can directly stimulate antibody production by B-cells called T-independent antigens.
3. Depending on the source, antigen are classified into following types;
1. Xenoantigens;
An antigen that is found in more than one species.
For example; bacteria, virus, parasite, fungi.
2. Alloantigens
These antigens vary with in the same species because of genetic variation and also include the human blood group antigen.
3. Autoantigen
It is an antigen from the body which when released from its location can stimulate an immune response to develop against it.
4. Heterophilic antigen
These are antigen which are closely related, even though they are from different species.
They are present in two different species but they share epitope with each other.
4. Depending on the basis of immunogenicity, antigens are classified into four different groups;
1. Immunogens
These are antigens that incluce specific immune response.
2. Telerogens
These antigen induce immunological tolerance, which is unresponsive to an antigen.
For example; our immune system generally tolerant of self antigens, so it does not usually attack the body's own cells, tissues and organs.
When tolerance is lost, disorder like autoimmune diseases occur.
3. Allergens
These antigens cause allergy or hypersensitivity reaction.
4. Antigen's vaccine
Some vaccines are madeup of only antigen which induce a protective response against microbes and are used to prevent disease.
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