The Differencies Between Antigens and Antibodies

The terms "antigen" and "antibody" are closely related, but they are far from interchangeable. We show you how they differ.

Written and verified by el biólogo Samuel Antonio Sánchez Amador.

Last update: 09 September, 2021

The human immune system is essential for our survival, as there are thousands of viruses and bacteria that try to colonize our internal environment every day in order to proliferate unlimitedly. Knowing the biological barriers that define us as a species is essential to explain pathological conditions, and this knowledge involves understanding the differences between antigens and antibodies.

Although they’re complementary terms, the words antigen and antibody refer to very different compounds. Both explain part of the immune response of humans, but one from the side of the pathogen and the other from the point of view of the host. If you want to know more about this topic, we encourage you to continue reading.

Overview of the immune system

Before going fully into the differences between antigens and antibodies, it’s relevant to take a brief tour of the human defenses. First of all, it should be noted that the National Cancer Institute (NIH) defines the immune system as ‘ the set of cells, tissues, organs, and substances that help the body fight infections and other diseases’.

It is postulated that this set of tissue networks exist in response to exogenous and endogenous aggressions. Viruses, bacteria, protozoa, helminths and fungi colonize the internal or external surfaces of the human being, but they always come from the environment. On the other hand, the cellular mutation that leads to cancer is endogenous, since its origin lies in the individual’s own genome.

At a general level, the immune system is classified as innate and acquired. The former has a series of cell bodies that respond nonspecifically to infections (phagocytes, macrophages, neutrophils and dendritic cells), while the acquired one recognizes a specific foreign element: the antigen. Lymphocytes are the typical cell bodies of the acquired response.

Innate and acquired immunity: two sides of the same coin

To this day, it is recognized that the “deterministic” view of this intricate biological fabric is not entirely correct. As indicated by the Elsevier medical portal, innate and acquired responses complement each other as follows:

1. The innate immune system activates the acquired one in response to infections. For example, macrophages respond nonspecifically, but present antigens to lymphocytes so that they act by setting up specialized mechanisms. This process is known as antigen presentation.
2. The acquired immune system uses the mechanisms of innate immunity to eliminate pathogenic microorganisms.

Thus, it can be ensured that both systems are complementary and help each other. We have already briefly introduced the term “antigen”, so we’re ready to differentiate it from antibodies in the following lines.

What are the differences between antigens and antibodies?

First of all, it should be noted that both the antigen and the antibody are biological compounds, that is, they’re present in living beings. Their differences lie in the role they play in the immune reaction, as we will see below. We dissect the disparities between both terms by parts.

1. The antigen belongs to the pathogen, and the antibody to the immune system

The United States National Library of Medicine defines antigen as ‘any substance that causes the synthesis of antibodies by the host’s immune system’. In other words, it’s a molecule present in the pathogenic organism (generally in its coat) that is recognized by immune cells and then promotes a defense response.

On the other hand, the same source defines the antibody as ‘a protein produced by the host’s immune system when it detects harmful substances (antigens)’. Each antibody, also called an immunoglobulin, recognizes a single molecule of the pathogen that has entered the body.

Thus, in a simple way, it could be said that the antigen is the “lock” provided by the threat, while the antibody is the “key” of the host that opens the door to an effective immune response. When both compounds are joined (by a type of chemical “connection” called noncovalent ), the antigen-antibody complex is formed.

2. The composition is not exactly the same

As Technology Networks indicates, another difference between antigens and antibodies lies in their chemical composition. For their part, antigens are peptides (short chains of amino acids), proteins and polysaccharides (chains of simple sugars). Some lipids and nucleic acids can become antigens, but only if they combine with proteins or polysaccharides.

On the other hand, antibodies are composed exclusively of proteins, without further additions. In later lines, we will see the three-dimensional conformation of this molecule, but for now it’s enough for us to know that each immunoglobulin contains constant and variable regions made up of chains of amino acids (the basis of all protein).

3. Antigens can be environmental or internal, but antibodies are always internal

Antigens can be efficiently classified according to their place of origin. The exogenous ones are those that come from sources external to the host’s organism by processes such as inhalation, direct ingestion, entry through wounds or injections, among many other routes. Once they enter the body, they are processed by macrophages and other cell bodies and presented to lymphocytes.

On the other hand, endogenous antigens are those that are produced by cellular metabolism or those that are expressed in a cell that has been infected by a virus or bacteria. In a way, autoantigens or “self-antigens” could be included in this category. As indicated by the Navarra University Clinic, the latter are compounds recognized as foreign when they shouldn’t be.

The recognition of an autoantigen as a foreign molecule favors the appearance of immune diseases. This is the case with rheumatoid arthritis, where the defense cell bodies attack and destroy the protein material present in the joints and establish chronic inflammatory mechanisms.

On the other hand, antibodies are always endogenous, since they’re produced within the host’s organism in response to the presence of antigens. These are synthesized in B lymphocytes, which in turn develop from hematopoietic stem cells present in the bone marrow.

4. The three-dimensional structure is very different

Another difference between antigens and antibodies is the three-dimensional conformation of both compounds. As we have said before, antigens are structures of a highly variable protein nature. However, they all have an epitope, a structure that’s recognized by B lymphocytes, antibodies, or T lymphocytes.

The structure of the antibody is much more standardized and has a typical “Y” shape. We’ll tell you its parts briefly in the following list:

• Each antibody or immunoglobulin is made up of 4 polypeptide sections: 2 heavy chains and 2 light chains identical to each other and connected by a disulfide bond. In turn, one part of the antibody is constant and another variable.
  • The variable part changes from one antibody to another and is the section that binds directly to the epitope of the antigen. The amino acid composition depends on the target of said antibody. It is represented by the end of the “Y” structure and contains a part of the heavy chains and another of the light ones.
  • The constant part is always the same within each category and, based on it, the different types of antibodies that exist (IgM, IgG, etc.) are determined. It represents the base of the “Y” and is only made up of heavy chains.
• The antigen-binding fragment is known as the Fab region. The crystallizable fragment, “antibody tail” or “base of the Y” is called the FC region.

The “Y” form cited is monomeric, but antibodies also occur as dimers and pentamers. These terms refer to the number of subunits that compose them. As you can see, the structure of the antigen is much simpler than that of the antibody.

5. The typology of each compound is different

There are different types of antigens and different types of antibodies, but the criteria used to classify them aren’t the same. We’ll now expand on this topic.

Types of antigens

Student documents classify antigens according to their origin as follows:

• Bacterial: This is an infectious antigen, as it’s carried by a pathogenic microorganism that comes from the outside. These are polysaccharides easily detected by the latex agglutination technique in organic fluids.
• Viral: This is also infectious, and, as its name implies, it’s found on the surface or covered with a virus.
• Protozoan and helminthic: These are antigens that also indicate infections, but on the part of somewhat more evolved living beings, such as protozoa, nematodes and flatworms. In general, these microorganisms tend to establish themselves in the intestinal region of the host.
• Autoantigen: From here on, the rest of the antigens to be cited are not infectious. As we have said in previous lines, these are compounds of the body itself that are mistakenly recognized as foreign agents and cause autoimmune diseases.
• Allergen: Allergens are antigens that trigger an exaggerated immune response in sensitive people after multiple exposures. They don’t represent a threat as such and don’t indicate an infectious picture, but the body is confused and interprets that it must fight them.

Types of antibodies

As you can see, antigens are usually classified according to their place of presentation, since a reaction caused by a virus is not the same as an allergic reaction caused by inhaling pollen. On the other hand, antibodies are categorized based on the heavy chain they present. These are the following:

• Immunoglobulin A (IgA): this type of antibody is predominant in the mucous membranes that line the respiratory tract and digestive system. It is also found in high concentrations in saliva, tears, and breast milk.
• Immunoglobulin G (IgG): as indicated by the KidsHealth portal, it is the most abundant type of antibody in the body. On this variant rests most of the protective action against pathogens and is the only one capable of crossing the placenta.
• IgD: occurs in small amounts in the bloodstream. By far, it is the least known antibody.
• IgM: this type of antibody is expressed on the surface of B lymphocytes or is free in the blood and has a very high specificity. It is the first variant to be synthesized to combat an infectious agent.
• IgE: this immunoglobulin is involved in inflammatory processes, especially allergic ones. It is also found in high concentrations when a parasite has infested the host’s body.

Summarizing this difference between antigens and antibodies, it can be concluded that antigens are classified according to the microorganism or compound they are found in, while antibodies or immunoglobulins (Ig) are characterized by their heavy chains. Each of the Ig has a series of specific tasks.

Antibodies and antigens: two sides of the same coin

After this extensive guide, we can see that the differences between antigens and antibodies are obvious. If we want one idea to be clear, it is the following: antigens are compounds present on the surface of pathogens, but sometimes they also come from allergenic compounds or from the human body itself. On the other hand, antibodies are always synthesized in the body.

Antigens are the agent that causes the immune reaction, while antibodies enable it. Both terms are two sides of the same coin, as they explain the intricate immune system that our species carries.