Antigen: Definition, Importance, Antibodies, Types and Investigations

They are any substance that stimulates the immune system to produce antibodies. They can be bacteria, viruses or fungi that cause infection and disease.

Antigens can also be substances, called allergens , that cause an allergic reaction. Common allergens include dust, pollen, animal dander, bee stings or certain foods.

Blood transfusions containing antigens incompatible with those in the body’s own blood will stimulate the production of antibodies, which can cause serious reactions that can be life-threatening.


Antibodies, also called immunoglobulins, are proteins made by the body that help fight foreign substances called antigens.

When an antigen enters the body, it stimulates the immune system to produce antibodies. (The immune system is the body’s natural defense system). The antibodies bind to the antigen and inactivate it.

Each body of a healthy adult has small amounts of thousands of different antibodies. Each one is highly specialized to recognize only one type of foreign substance.

The antibody molecules are typically Y-shaped, with a binding site on each arm of the Y. The binding sites of each antibody, in turn, have a specific shape.

Only the antigens that match this shape will fit into them. The role of antibodies is to bind with antigens and inactivate them so that other bodily processes can control, destroy and eliminate foreign substances from the body.

Importance of antigens

The important antigens in practical immunology are not mainly dispersed molecules but cellular or multicellular structures. Of course, it is finally definable in all cases in molecular terms.

Viruses, sometimes even crystallizable, are among the simplest structures, sometimes of a purely protein nature, but often include lipids or polysaccharides.

In some cases, the inner core proteins are also efficient antigens, capable of providing a protective immune response.

We have learned more about immunology from studies on bacteria than from any other group of natural antigens, but they also involve proteins and teichoic acids.


Vaccines used in medical practice include four main types of antigen preparations:

  • Toxoids (the soluble exotoxins of bacteria such as diphtheria and tetanus bacillus, which have been modified and have become less toxic by the addition of formalin or mild heating).
  • Antigens isolated from infectious agents (such as the capsular polysaccharide of pneumococci).
  • Dead vaccines (organisms killed by heat, ultraviolet radiation or chemical products such as phenol, alcohol or formalin).
  • Live attenuated vaccines (made from strains of organisms that have lost their virulence due to growth in culture).

The allergens

Allergens are antigens that cause allergic reactions of the immediate or delayed type. They can be of very different origins, such as dust, fungi, hair, pollen, bacterial proteins, food or drugs.

The immediate-type allergy is induced mainly through a mechanism triggered by IgE-class antibodies, whereas the delayed-type allergy is mediated by T cells.

All plant and animal cells possess antigens that can be expressed in a foreign host. Many animal and human antigens can trigger autoimmune phenomena.


Some antigens may be specific for one organ, while others are present essentially in all cells (eg, histocompatibility antigens).

In the last two decades, the central role of the MHC in immunological processes was recognized. MHC, a group of diverse genes that mediate and regulate a variety of immune mechanisms, appears to exist in all higher vertebrate species (HLA in humans, H2 in mice).

The genetic, structural and functional studies of the multiple MHC products have defined three broad classes of genes and molecules. The products of class I are glycoproteins expressed in the membranes of all nucleated cells (histocompatibility or transplantation, antigens).


These are the main objectives of the graft rejection reaction, and mediate the recognition and destruction of virus-infected or neoplastic cells.

Class II products, defined by immune response genes (Go for its acronym in English), are expressed mainly in the membranes of antigen-presenting cells (eg, macrophages, dendritic cells, B lymphocytes ) and mediate regulation.

Through the so-called auxiliary and suppressive effects, a variety of humoral and cellular immune responses.

When they are incompatible, these Ir gene products also play an enhancing role in transplant rejections.

Class III genes determine the structures of several discrete proteins that include certain components of the complement system that cause destruction and elimination of bacteria and other foreign cells.

Classes of antibodies and their functions

There are five classes of antibodies, each with a different function. They are IgG, IgA, IgM, IgD and IgE. Ig is the abbreviation for immunoglobulin or antibody.

IgG antibodies are the most common and the most important. They circulate in the blood and other bodily fluids, defending against the invasion of bacteria and viruses.

The binding of IgG antibodies to bacterial or viral antigens activates other immune cells that engulf and destroy antigens. The smallest of the antibodies, IgG moves easily through the cell membranes.

In humans, this mobility allows IgG in a pregnant woman to pass through the placenta to her fetus, providing a temporary defense to her unborn baby.

IgA antibodies are present in tears, saliva and mucus, as well as secretions from the respiratory, reproductive, digestive and urinary tracts. IgA works to neutralize bacteria and viruses and prevent them from entering the body or reaching the internal organs.

IgM is present in the blood and is the largest of the antibodies, combining five units in the form of Y. It works in a similar way to IgG in the defense against antigens, but it can not cross the membranes due to its size.

IgM is the primary antibody produced in an initial attack by a specific bacterial or viral antigen, whereas IgG usually occurs in subsequent infections caused by the same agent.


Allergen: Foreign substance that causes an allergic reaction in the body.

B cells: Cells produced in the bone marrow that secrete antibodies.

Immune response: the production of antibodies in response to foreign substances in the body.

Immunity: the condition of being able to resist the effects of a particular disease.

Immunization: the process of making a person capable of resisting the effects of specific foreign antigens.

Inoculate: Introduce a foreign antigen into the body to stimulate the production of antibodies against it.

Monoclonal antibodies: identical antibodies produced by cells cloned from a single cell.

Proteins: large molecules that are essential for the structure and functioning of all living cells.

Vaccine: Preparation of a live microorganism weakened or dead from a particular disease administered to stimulate the production of antibodies.

IgD: is present in small amounts in the blood. This class of antibodies is found mainly on the surface of B cells, cells that produce and release antibodies. IgD helps B cells recognize specific antigens.