What is the Difference Between an Antigen and a Hapten

The main difference between an antigen and a hapten is that an antigen is a complete molecule that can trigger an immune response by itself whereas a hapten is an incomplete molecule that cannot trigger an immune response by itself. 

Antigen and hapten are two types of immunogens that can trigger immune responses. Furthermore, Antigens can bind to the MHC complex while hapten cannot bind to the MHC complex and, cannot be presented to the T cells.

Key Areas Covered

1. What is an Antigen
     – Definition, Features, Types
2. What is a Hapten
     – Definition, Features, Hapten-Carrier Adduct
3. What are the Similarities Between an Antigen and a Hapten
     – Outline of Common Features
4. What is the Difference Between an Antigen and a Hapten
     – Comparison of Key Differences

Key Terms

Antibody Response, Antigen, Hapten, Hapten-Carrier Adduct, MHC Complex

What is an Antigen

An antigen is a molecule that can trigger an immune response by acting as an immunogen. It can be either a protein, peptide or polysaccharides. Lipids and nucleic acids can also serve as antigens when binding to proteins. A particular antigen may contain one or more epitopes, which are the antigen determinants. Antibodies recognize and bind to these epitopes. Furthermore, the immune system produces specific glycoproteins called antibodies in response to epitopes. There are four main types of antigens occur in the body:

• Exogenous antigens – Epitopes on the surface of the pathogens found in the extracellular space serve as exogenous antigens.

Figure 1: Antigens Directly Bind to an Antibody

• Endogenous antigens – Epitopes generated inside the cell as a result of the cell metabolism are endogenous antigens. 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 – These are the endogenous antigens mistakenly recognized as non-self antigens by the immune system. This results in the destruction of self-tissues, causing autoimmune diseases.
• Neoantigens – These are the molecules expressed on the surface of the cells infected by oncogenic viruses.

What is a Hapten

A hapten is an incomplete antigen which can elicit an immune response only when attached to a large carrier such as a protein. Hapten carriers are circulating molecules through the body. An adduct refers to the combination of the hapten and the carrier. However, haptens cannot bind to the MHC complex independently; hence, they cannot be presented to the T cells. Urushiol is a well-known hapten found in Ivy. It serves as a toxin, which causes cell-mediated contact dermatitis. When absorbed, it is oxidized into quinone inside the skin cells. Quinone reacts with skin proteins to form an adduct.

Figure 2: Formation of Hapten-Carrier Adduct

The hapten-carrier adduct serves as a complete antigen, which can trigger an immune response. Our body produces antibodies in response to the hapten-carrier adduct. However, hapten can inhibit the binding of the antibody to the hapten-carrier adduct in a process known as hapten inhibition, inhibiting the antibody response.

Similarities Between an Antigen and a Hapten

• Antigen and hapten are two molecules, which can trigger immune responses by acting as antigenic agents.
• Moreover, they can serve as immunogens.
• Also, both antigens and haptens can bind to antibodies.

Difference Between an Antigen and a Hapten

Definition

An antigen refers to a toxin or other foreign substance which induces an immune response in the body, especially the production of antibodies, while a hapten refers to a small molecule which, when combined with a larger carrier such as a protein, can elicit the production of antibodies that bind specifically to it (in the free or combined state). These two definitions explain the basic difference between an antigen and a hapten.

Complete or Incomplete Antigens

Antigens are complete molecules while haptens are incomplete antigens. Hence, this is a major difference between an antigen and a hapten.

Binding to MHC Complex

Furthemore, the ability to bind to MHC complex contributes to a difference between an antigen and a hapten. That is; antigens can bind to the MHC complex while haptens cannot bind to the MHC complex.

Binding to Antibodies

Another difference between an antigen and a hapten is that antigens can directly bind to antibodies while haptens cannot directly bind to the antibodies.

Immune Response

In addition, an important difference between an antigen and a hapten is that antigens can trigger an immune response by themselves while haptens cannot trigger an immune response by themselves.

Conclusion

In brief, antigens are molecules that can elicit an immune response by themselves whereas haptens need to bind to a carrier molecule to become a complete antigen, which elicits the immune response. Both antigen and hapten-carrier adduct serve as immunogens. Thus, the main difference between an antigen and a hapten is the ability to elicit an immune response independently.

Reference:

1. “Antigens.” Lumen|Boundless Anatomy and Physiology, 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