The key difference between epitope and hapten is that epitope is the part of an antigen that is recognized by an antibody, while hapten is a small molecule that can elicit an immune response only when conjugated to a suitable carrier protein.
The way the human body defends itself against harmful pathogens or foreign elements is called an immune response. In an immune response, the immune system recognizes the antigens on the surface of the infectious agents, attacks and destroys them using macrophages or antibodies. Epitope and hapten are two important structures involved in the immune response process.
What is an Epitope?
Epitope or antigenic determinant is the part of an antigen that is recognized by an antibody in order to trigger an immune response. An epitope is specifically recognized by antibodies, B cells, or T cells. The part of the antibody that binds to the specific epitope in the antigen is called a paratope. Epitopes are usually non-self proteins. However, sequences that are derived from the host that can be recognized also function as epitopes in the case of autoimmune diseases.
The epitopes of antigens are divided into two categories as conformational epitopes and linear epitopes. This division is based on their structure and interaction with the paratope. The conformational epitope is normally formed by the 3D conformation adopted by the interaction of discontiguous amino acid residues. On the other hand, a linear epitope is normally formed by the 3D conformation adopted by the interaction of contiguous amino acid residues. Moreover, a linear epitope is not determined solely by the primary structure of the involved amino acids. Furthermore, 90 % of epitopes are conformational, and the rest of the 10 % are linear in nature. Epitope-based vaccines were first developed in 1985.
What is Hapten?
A hapten is a small molecule that can elicit an immune response only when conjugated to a suitable carrier protein. Hapten reacts with a specific antibody, but it is not immunogenic by itself. Moreover, hapten can only be made immunogenic after conjugation to a suitable carrier protein-like antigen. After hapten is bound to a larger molecule like a carrier protein, it will become a complete antigen. Therefore, a hapten is essentially an incomplete antigen. Many drugs like penicillin are haptens.
A carrier does not elicit an immune response by itself. When the body develops antibodies to a hapten-carrier adduct, the small hapten molecule may also be able to bind to the antibody. But only the hapten-carrier adduct usually initiates the immune response. Furthermore, hapten molecules can sometimes even block immune responses to the hapten-carrier adduct from binding to the antibody. This process is called hapten inhibition.
What are the Similarities Between Epitope and Hapten?
- Epitope and hapten are two important structures involved in the immune response process.
- Both can bind to the antibodies.
- They can be identified in the blood.
- Both are very important for the human body to defend against pathogens like bacteria, viruses, or other toxins.
What is the Difference Between Epitope and Hapten?
An epitope is the part of an antigen that serves as an antigenic determinant recognized by an antibody, while a hapten is a small molecule that can elicit an immune response only when conjugated to a suitable carrier protein. Thus, this is the key difference between epitope and hapten. Furthermore, epitope is a portion of a foreign protein or self protein, while hapten is an incomplete antigen.
The below infographic presents the differences between epitope and hapten in tabular form for side by side comparison.
Summary – Epitope vs Hapten
Epitope and hapten are two important structures needed in the creation of an immune response. Epitope is the antigenic determinant that is recognized by an antibody, while hapten is a small molecule that can elicit an immune response only when conjugated to a suitable carrier protein. So, this summarizes the difference between epitope and hapten.
1. “Epitope-mapping illustration-6-copy” By Kathryn Edmondson (as employee of Integral Molecular) – Own work, (CC BY-SA 4.0) via Commons Wikimedia
2. “Hapten” By MantOs – Own work (CC BY-SA 3.0) via Commons Wikimedia