Tuesday, 20 July 2021

Concept of Antigenicity and Immunogenicity

Here we will understand about two more confusing terms in immunology. These are antigenicity and immunogenicity. To understand these terms we should have an idea about how adaptive immune response works.

How adaptive immune system works?

  Main cells of adaptive immune system are known as lymphocytes. The two important lymphocytes are B-cells and T-cells. B-cells deal with extracellular pathogens and the T-cells deal with intracellular pathogens.

Both of these cells are produced in the bone marrow. Although B lymphocytes complete their maturation in the bone marrow itself. T lymphocytes travel to the organ called thymus where they finally mature.

After the maturation stage, These cells are released into the blood they circulate in blood and have ability to enter tissues where they can recognise pathogens, if present. For recognition purpose these cells possess specific receptors. which we know as B-cell receptors and T-cell receptors.

When this happens these lymphocytes are stimulated to respond and mount a specific immune response.
   When B cells recognize an antigen they get activated and get differentiated into plasma cells. These plasma cells produce antibodies specific to that antigen.
  The antibodies bind to those antigens and activate defense mechanisms that lead to the destruction of the pathogen. Some of the activated B-cells remain in circulation as memory B-cells.
This immune response is known as humoral immune response.

Similarly when T cells recognise the antigen. They differentiate into T cell subsets namely T helper cells and T cytotoxic cells. These subsets of T cells further lead to the destruction of the invader and the infected cells.
  Some of these cells also differentiate into memory T cells. This immune response is known as cellular immune response.

Immunogenicity and antigenicity

When pathogen such as bacteria say invaded the body, here bacteria are the antigens. once inside the body B-cells recognise these antigens or more specifically antigenic determinants or epitopes of the bacteria. After recognition of the antigen B cells differentiate into plasma cells which are specific to this antigen.

  These plasma cells produce specific antibodies which bind to the antigens and further lead to the destruction of that pathogen. So Substance and antigen induced an immune response.

The ability of a substance to induce an immune response, that is either humoral or cellular immune response is known as immunogenicity. And such substances are called immunogenic.

Now suppose a related but harmless bacteria with similar epitopes invades the body. This time since antibodies specific to the epitopes of this bacteria are already present inside the body. These antibodies bind to invaded bacteria.

" here note that the bacteria does not induce an immune response it just binds to product of an immune response that is antibodies."

So the ability of a substance to bind specifically to the products of an immune response is known as antigenicity. And such substances are called antigenic.

Definition of immunogenicity and antigenicity

Immunogenicity is defined as the ability of a substance to induce an immune response. Whereas antigenicity is defined as the ability of a substance to bind specifically to the products of the immune response.

"All molecules that are immunogenic are antigenic too."

All molecules which are able to induce an immune response are also able to bind to the final products of the immune response.

Humoral immune response was induced by the antigen (bacteria) and the antibodies produced bind specifically to that antigen. So substances which are immunogenic are also antigenic.

"All antigenic molecules cannot be considered immunogenic."

There are some substances which are able to bind to the final products of an immune response. That means they are antigenic but they cannot induce an immune response by themselves.
  Well-known example of such substances or haptens. Haptens are able to react with the antibodies, but they are unable to stimulate their production.

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