What is the Difference Between Antigen and Immunogen

The main difference between antigen and immunogen is that antigen is any structure that binds to the components of the immune system, including antibodies, B cells, and T cells, whereas immunogen is a type of antigen capable of inducing an immune response. 

Antigen and immunogen are two types of molecules that bind to the components of the immune system. Antigens can be either proteins, polysaccharides, lipids or nucleic acids while immunogens are normally proteins and large polysaccharides. However, lipids and nucleic acids can also be immunogenic after binding to proteins or large polysaccharides. 

Key Areas Covered 

1. What is an Antigen
     – Definition, Structure, Importance
2. What is an Immunogen
     – Definition, Structure, Importance
3. What are the Similarities Between Antigen and Immunogen
     – Outline of Common Features
4. What is the Difference Between Antigen and Immunogen
     – Comparison of Key Differences

Key Terms 

Antibody, Antigen, Immune Response, Immunogen, Immunogenicity

What is an Antigen 

An antigen is a molecule that can bind to the components of the immune system, including antibodies, T cells, and B cells. Generally, antigens can be either proteins, peptides, polysaccharides, lipids or nucleic acids. A particular antigen may contain one or more epitopes or antigen determinants. Normally, epitopes are parts of an antigen. Moreover, the complementary parts of epitopes which occur on antibodies are called paratopes. 

Figure 1: Antigens

Furthermore, there are four main types of antigens that occur in the body. They are: 

• Exogenous antigens – Antigens on the surface of pathogens found in the extracellular space 
• Endogenous antigens – Antigens generated inside the cell as a result of the cell metabolism (The epitopes generated as a result of the regular metabolism are self-antigens while the molecular components of the pathogens inside cells are non-self antigens.) 
• Autoantigens – Endogenous antigens mistakenly recognized as non-self antigens by the immune system (This results in the destruction of self-tissues, causing autoimmune diseases.) 
• Neoantigens – Antigens expressed on the surface of the infected cells by oncogenic viruses 

What is an Immunogen 

An immunogen is a type of antigen which can trigger an immune response. Generally, all antigens are not immunogenic. Moreover, immunogens are the immunogenic antigens. Basically, proteins, peptides, and large polysaccharides are immunogenic. However, lipids and nucleic acids can also serve as immunogens after binding to proteins or polysaccharides. On the other hand, immunogens can trigger either humoral or cell-mediated immune responses. Furthermore, the ability of immunogens to trigger such immune responses is known as immunogenicity. 

Figure 2: Haptens

Moreover, one can define an immunogen as a complete antigen made up of an epitope and a macromolecular carrier molecule. For example, the hapten is a low-molecular-weight compound that can bind to an antibody. However, it is unable to generate an immune response by itself. Nevertheless, when a hapten binds to a nonimmunogenic carrier, the hapten-carrier complex can evoke an immune response. 

Similarities Between Antigen and Immunogen 

• Antigen and immunogen are two types of molecules that bind to the components of the immune system, including antibodies, B cells, and T cells. 
• Moreover, they can be either proteins, polysaccharides, lipids or nucleic acids. 

Difference Between Antigen and Immunogen 

Definition 

An antigen refers to a substance specifically bind to antibodies or a cell surface receptors of B cells and T cells while an immunogen refers to an antigen capable of inducing an immune response. Thus, this is the main difference between antigen and immunogen.

Immunogenicity 

Another difference between antigen and immunogen is that antigens can be either immunogenic or non-immunogenic while immunogens are immunogenic. 

Correspondence 

Not all antigens are immunogens, but all immunogens are antigens. 

Type of Molecules 

Antigens can be either proteins, polysaccharides, lipids or nucleic acids while immunogens are normally proteins and large polysaccharides. 

Haptens 

Haptens are low-molecular-weight molecules which bind to antibodies while haptens become immunogenic when binding to larger carrier molecules. 

Conclusion 

An antigen is a molecule which can be either a protein, polysaccharide, lipid or nucleic acid. Moreover, it can bind to antibodies and the receptors on T cells and B cells. However, not all antigens are immunogenic. On the other hand, immunogenic antigens are known as immunogens. Generally, proteins and large polysaccharides are immunogenic, generating an immune response. Meanwhile, lipids and nucleic acids can become immunogenic after binding to proteins or large polysaccharides. Therefore, the main difference between antigen and immunogen is their immunogenicity. 

References:

1. McDuffie F.C. (1981) Antigens and Immunogens. In: Safai B., Good R.A. (eds) Immunodermatology. Comprehensive Immunology, vol 7. Springer, Boston, MA. Available Here.

Image Courtesy:

1. “Antibody” By Fvasconcellos 19:03, 6 May 2007 (UTC) – Color version of Image:Antibody.png, originally a Work of the United States Government (Public Domain) via Commons Wikimedia   
2. “Hapten” By MantOs – Own work (CC BY-SA 3.0) via Commons Wikimedia