What is the Difference Between Hapten and Adjuvant

The main difference between hapten and adjuvant is that hapten is a low molecular weight antigen that is not antigenic only when it binds to a large carrier molecule such as a protein, whereas adjuvant enhances the immunogenicity of a vaccine when mixed with an antigen.

Hapten and adjuvant are two types of molecules important in immunogenicity. Generally, a hapten needs large molecules as carriers to form a hapten-carrier adduct while an adjuvant boosts the immunogenicity of a vaccine.

Key Areas Covered

1. What is Hapten
   • Definition, Structure, Function
2. What is Adjuvant
   • Definition, Structure, Function
3. Similarities Between Hapten and Adjuvant
   • Outline of Common Features
4. Difference Between Hapten and Adjuvant
   • Comparison with Key Differences

Key Terms

Adjuvant, Hapten, Hapton-Carrier Adduct

What is Hapten

A hapten is a small molecule that only elicits an immune response when attached to a large carrier molecule such as a protein. Moreover, the carrier molecule does not elicit an immune response itself. In general, large molecules, insoluble foreign matter, and infectious agents can only elicit an immune response. When the hapten binds to the carrier molecule, the complex that forms is known as a hapten-carrier adduct. The hapten-carrier adduct can elicit an immune response. After the generation of antibodies, the hapten is capable of binding to the antibody. Sometimes, hapten blocks the binding of the hapten-carrier adduct to the antibodies, blocking the immune response. This process is known as hapten inhibition.

Figure 1: Hapten

For instance, penicillin is a hapten, a small molecule that is not antigenic. However, it combines with serum proteins of sensitive individuals and the resulting molecule is immunogenic. Usually, the resulting molecule complex activates lymphocytes and the immune response initiated can be a fatal allergic reaction.

What is Adjuvant

In immunology, an adjuvant is a substance that increases immunogenicity: the immune response to a vaccine.  It also acts to accelerate, prolong, or enhance antigen-specific immune response when used in combination with specific vaccine antigens. Therefore, it boosts the immune response of the vaccine. Normally, this is useful when the antigen has low immunogenicity and a small amount of antigen is present. Moreover, adjuvants may modify the immune response of a vaccine.

Figure 2: Vaccine Booster

As an example, when the Bovine Serum Albumin (BSA) immunization is given with an adjuvant, its immune response may increase five-fold. Ammonium salts, oils, and virosomes are some of the other types of adjuvants.    

Similarities Between Hapten and Adjuvant

• Hapten and adjuvant are two substances that can initiate an immune response in different methods in immunology.
• Alone, both can not elicit an immune response.

Difference Between Hapten and Adjuvant

Definition

Hapten refers to a small molecule that, when combined with a larger carrier such as a protein, can elicit the production of antibodies that bind specifically to it, while adjuvant refers to a substance that enhances the body’s immune response to an antigen.  

Significance

Generally, hapten binds to larger carrier molecules to initiate an immune response while adjuvant can boost the immunogenicity of a vaccine.  

Examples

One example of a hapten is penicillin while examples of adjuvants are ammonium salts, oils, and virosomes.

Conclusion

In brief, a hapten is a small molecule with low molecular weight. It cannot initiate an immune response alone. Therefore, it needs to bind to a large carrier molecule such as a protein to initiate an immune response. In addition, hapten can block an immune response by blocking the binding of the carrier molecule to antibodies in a process called hapten inhibition. In contrast, an adjuvant is a substance that boosts the immunogenicity of a vaccine. Some examples of adjuvants include ammonium salts, oils, and virosomes. Therefore, the main difference between hapten and adjuvant is their function in the immune system.

References:

1. Wikimedia Foundation. (2022, March 5). Hapten. Wikipedia. Retrieved August 9, 2022.
2. Wikimedia Foundation. (2022, May 23). Immunologic adjuvant. Wikipedia. Retrieved August 9, 2022.

Image Courtesy:

1. “Hapten ” By MantOs – Own Work (CC BY-SA 3.0) via Commons Wikimedia
2. “Vaccine Booster ” By The Focal Project – Own Work (CC BY-NC 2.0) via Flickr