What is a Hapten?

Mary McMahon
Mary McMahon

A hapten is a small molecule which cannot act as an antigen independently, but will stimulate the production of antibodies if it manages to find a carrier to bind to. Haptens are involved in the production of a number of immune responses, making them a topic of interest to immunologists, and they are also used in laboratory research and some types of diagnostic testing. People have been researching these small molecules since the early 1900s.

An example of hapten can be found in poison oak.
An example of hapten can be found in poison oak.

Haptens are also known as incomplete or partial antigens, and alternate spellings include “haptene” and “haptin.” The term “hapten” comes from a Greek word which means “to bind,” referencing the idea that these molecules have to bind with other molecules in order to become functional. These molecules must find protein molecules which can act as carriers in order to become an antigen which the immune system will be able to recognize.

Coming in contact with poison oak can cause red, itchy blisters to appear at the site of contact.
Coming in contact with poison oak can cause red, itchy blisters to appear at the site of contact.

A classic example of a hapten can be found in poison oak. When people are exposed to poison oak or related plants for the first time, the hapten enters their skin, oxidizes, and binds with the proteins it finds. The immune system recognizes the hapten once it binds to a protein and develops specific antibodies which will react the next time the body is exposed to poison oak. The immune response classically causes the appearance of an itchy red rash which can be quite uncomfortable.

Researchers have conducted studies to find the types of molecules which haptens bind to so that they can learn more about them. This research is also used in the preparation of specific antigens in the laboratory environment, and it can be useful in immune assays which are designed to test a patient's immune system or to look for specific antibodies and antigens which may be present in the body. Thousands of haptens have been discovered and mapped, and several databases of haptens and carriers are available to people who work with these molecules.

Understanding haptens is part of the larger study of immunology, the study of the immune system and how it works. Haptens are part of the system known as humoral immunity, which revolves around the development of antibodies which have learned to recognize specific antigens. The processes involved in humoral immunity can get very complex, and the immune system has been known to get overzealous about antibody production, developing antibodies to substances which are not actually harmful, such as the proteins found in peanuts.

Mary McMahon
Mary McMahon

Ever since she began contributing to the site several years ago, Mary has embraced the exciting challenge of being a wiseGEEK researcher and writer. Mary has a liberal arts degree from Goddard College and spends her free time reading, cooking, and exploring the great outdoors.

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Discussion Comments


@indigomoth - Not all allergies are caused by haptens, but some of them are. However, you wouldn't want to block them from joining to proteins altogether, because in some cases you absolutely want that immune response to an invader.

While allergies can be dangerous and certainly they are uncomfortable, they are a small price to pay for a system that generally does an excellent job at protecting us from dangerous bacteria and other body invaders that could otherwise quickly lay us flat.

Which is probably why it's so difficult to permanently combat allergies. After all, we know how to destroy the immune system.

Making it work just when we want it to work is a different matter.


Peanut allergies can be absolutely devastating. I didn't know they were associated with hapten antibodies.

I knew though that it was a result of the body overreacting to a certain kind of chemical.

Honestly, it just seems weird that there are so many things that can kill a person through allergies, or make his or her life miserable, which don't really appear to have any purpose.

With poison oak, I kind of understand it, as it could be a defense mechanism that the plant developed. But most allergies seem to have no purpose at all, from any point of view.

Here's hoping that at some point they manage to discover how to block the wrong haptens from joining with proteins altogether.

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