Antigen

What is an Antigen

Immune System | Biotechnology

 

After learning about the various basic conceptual of immune system, today we will go through into the deep learning about an Antigen.

An introductory statement for an Antigen has been pen down in my post ‘Immune System Basic Concepts’. I would like to advise all of you to please go through the above post, so that you can at least get an idea about the word Antigen.

Here, we all will be going through the various aspects of an Antigen, which will cover the following queries.

·        What is an Antigen?

·        What are Immunogenicity and Antigenecity and how they are different from each other?

·        What are Epitopes?

·        What are Haptens and how they are related to Antigenecity?

·        What are Pattern Recognition Receptors?

Let’s begin our learning process step by step…

·        What is an Antigen?

An Antigen is simply a protein molecule expressed by a bacteria or virus, which is recognized as an invader or a foreign body by the immune system, which in turn stimulates the production of antibody to specifically combine with them.

fig. Antigen

Basically, Antigen is present on the cell surface of bacteria and viruses.

Antigen comprises of bacteria, cells of transplanted organs, pollen particles of plants and toxins.

·        What are Immunogenicity and Antigenecity and how they are different from each other?

Immunogenicity is the term used when a foreign molecule is capable enough to generate a humoral immune response leading to the production of antibodies.

It can be better explained with an example: If B cells get activated , it will differentiate into plasma B cells, resulting in secretion of antibodies.

Antigenicity or Reactogencity is the process which defines the ability of combining with the final products of humoral or cell mediated immune response.

It does not activate the immune response; rather it combines with the final products of the immune response.

For clear understanding about antigenicity, we will cite an example: On activation of B cells, it secretes antibodies and antigen having the property of antigenecity will combine with the respective antibodies.

Difference between Immunogenicity and Antigenecity :

A single statement is sufficient to explain or clear the confusion between Immunogenecity and Antigenecity.

It says “All molecules having property of immunogenicity , also possess the property of antigenecity, but the reverse is not true i.e. molecules having antigenecity does not necessarily have the property of immunogenicity ”.

·        What are Epitopes?

Epitopes are the parts or portions or regions of antigen which are responsible for the specific character of antigen during antigen- antibody (Ag-Ab) reaction.

Epitopes combine with the complementary antigen binding site of antibody i.e. A single individual antibody recognize a single epitope.

Polyclonal antibodies are an exception to the above mentioned statement about epitopes, as these antibodies recognize many different epitopes.

fig. Epitope

Epitopes are divided into two types :

i.                   Linear Epitopes

ii.                 Discontinuous Epitopes

i.                   Linear Epitopes:

Linear Epitopes constitute the residues of amino acids which are adjacent in the polypeptide chain.

ii.                 Discontinuous Epitopes :

Discontinuous Epitopes are created from amino acid residues which are located in different parts of the polypeptide chain.

Linear Epitopes and Discontinuous Epitopes can be diagrammatically represented as shown in the below figure.

fig. Types of Epitopes

Epitopes are distinguished into three types :

a.     Free soluble molecule

b.     Surface bound molecule

c.      Degraded fragments

B cell receptors bind with any of the above mentioned category of epitopes  i.e.  they can be either free soluble molecule or surface bound molecule  and might be degraded fragments.

On the other hand, T cell receptors bind only to the epitopes which are intact with the host cell surface molecule.

·        What are Haptens and how they are related to Antigenecity :

Haptens or Incomplete Antigens or Artificial monovalent epitope are minute, non biologic molecules like peptides or nucleotides which provoke an immune response, only when attached to a large carrier.

fig. Haptens

When haptens link up with body proteins, the body’s adaptive immune response recognize them as foreign particle and mount an attack.

Haptens are used in Antibiotics and Anesthetics designing.

They are present in poison ivy, some detergents and cosmetics.

Haptens are of two types :

i.                   Complete haptens

ii.                 Incomplete haptens

i.                   Complete Haptens :

Complete Haptens when combine with specific antibody forms precipitates.

ii.                 Incomplete Haptens :

Incomplete Haptens on combining with specific antibody, no precipitation is formed.

‘Haptens have the property of antigenecity but not immunogenicity’. This statement can be well explained by the famous Land Steiner’s Experiment.

i.                   Hapten                    à Injected in mice à No immune response

(2,4- dinitrophenyl tyrosine)

ii.                 Carrier (protein)   à Injected in mice à Positive immune response

iii.              Hapten- Carrier Conjugate à Injected in mice à Anti hapten and anti

Carrier antibodies

·         What are Pattern Recognition Receptors?

They are the immune system receptors that recognize the structural motifs of the pathogen.

Pattern Recognition Receptors work in the following way:

a.     Firstly, they eat the pathogens and break down their protein into smaller particles.

b.     These protein fragments are transferred to their cell surface with the help of  Major Histocompatibility Complex (MHC) molecule.

c.      On activation, they switch on the cell surface receptor, which in turn activates T cells and B cells, further shaping up the immune response.

Examples of most known Pattern Recognition Receptors are:

Ø Mannose Binding Lectin (MBL)

Ø Macrophage Mannose Receptor

Ø Scavenger Receptors

Ø Toll like Receptors (TLR’s)

Ø RIG – I like Receptors (RLR’s)

Ø Nod like Receptors (NLR’s)

After going through the descriptive learning, now we will move towards some memorizing points.

Points to Remember :

ü An antigen is a protein, considered as foreign by the immune system and produce antibodies.

ü Immunogenicity is ability to produce humoral immune response.

ü Antigenicity is the process of combining with the final products of humoral or cell mediated immune response.

ü Epitopes are portions of antigen which combine with the binding site of antibody.

ü Haptens are  molecules which produce immune response, when attached to a big carrier.

ü Pattern Recognition Receptors are the receptors which recognize structural design of pathogens.

I hope, all your queries must be resolved, after discussing about “Antigen” .

For a proper command on the topic, I would like to suggest all of you to please go through all my posts at my blog "Learn Biotechnology with DeepaliTalk".

Starting from the very beginning to this post, I have discussed each and every topic in such a way that it becomes easier for you to understand every topic step by step.

Before taking your leave, I would like to advise you all on my professional expertise that “Keep calm and don’t panic”.

I know it seems to be quite complex for you to understand, but believe me all of this becomes very very easy, once we will start discussing each one of these topics separately. Have patience and don’t give up and just understand the topics as they are without any stress and please remember “Don’t get confused with the similar sound words”.

There is lot more to share, Keep in touch and stay happy.

Happy Antigen Learning!!!