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13.3.2. Concept and nature of antigens

Concept and nature of antigens

An antigen is a substance that triggers the formation of antibodies and can cause a specific immune response. The modern definition encompasses all substances that can be recognized by the adaptive immune system, whether they are its own or other people's.

Antigens are macromolecules that can be found free or on the cell surface of the pathogen, generally proteins (independent or linked to carbohydrates or lipids) and polysaccharides.

For the immune system to trigger a response, it is necessary for antigens to bind to antigenic receptors located on the plasma membrane of lymphocytes. They bind by an area of ​​the antigen, called the epitope or antigenic determinant.

The antigen is univalent when it has a single epitope in its molecule and, therefore, only one antibody can bind to it, while if it is polyvalent, it presents several antigenic determinants.

There are some low molecular weight molecules, foreign to the body, that cannot provoke an immune response, but if they bind to a carrier protein, such as albumin, they stimulate an immune response, behaving like antigens. These molecules are called haptens.

According to the origin, antigens are classified into:

  • Heteroantigens or xenoantigens. They are molecules that belong to organisms of another species (microorganisms) different from that of the receptor. They are molecules located in viral capsids or envelopes, in bacterial walls, on the surface of cells or in molecules secreted by them, such as toxins.
  • Isoantigens. They are molecules that belong to another individual of the same species, such as the surface antigens of the red blood cells that make up the AB0 system.
  • Self antigens. They are macromolecules of the body itself that, abnormally, the immune system recognizes as foreign and therefore considers antigens. Thus, the immune system acts against its own organism, in the phenomenon called autoimmunityIt should not happen, since the immune system has the ability to recognize its own molecules and differentiate them from foreign ones.

When we eat, proteins are digested in the digestive tract and pass into the blood as amino acids without producing any reaction. If they were to enter intravenously directly, with all their structures unaltered, they would be antigens and would cause the formation of antibodies.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Aragon. June 2015, option B, question 2.

Immunity: (2 points)

a) It is known that the immune system reacts against all kinds of molecules that it does not recognize as its own.

a.1. What are these molecules called? (0.3 points)

a.2. What is the reason why we reject proteins that are injected into us intravenously while if we take them digestively, they generally do not provoke the response of our immune system? Reason for the answer. (0.7 points)

Madrid, June 2018, option B, question 2.

In relation to antigens:

a) Define them and indicate their chemical nature (0.5 points).

b) Cite the name of the binding sites of the antigen and the antibody respectively (0.5 points).

c) Name and briefly describe the types of antigen-antibody reaction (1 point).

Fundamental ideas about antigens

The antigens are molecules, usually proteins (independent or bound to sugars or lipids) and polysaccharides, generally outside the body, capable of triggering a immune response.

The antigen has an area called the epitope or antigenic determinant, which is why it binds to the antigenic receptors located on the plasma membrane of lymphocytes. The spatial shape of the receptors and antigens are complementary. 


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