Re: why are people allergic to penicillin? What is an alternative?

Dear Sara-

This is a great question, and the real answer is that we don't know why. However, there are a couple of reasons why allergy to penicillin could be "built in" to a person's immune response. Penicillin allergy is among the most common of drug allergies,and usually produces rash and a fever, but can be fatal in some people (1,2). But, back to your question: why are people allergic to penicillin?

You may be aware that your body has a special system set up, known as the immune system, who's whole job is to keep you healthy by figuring out what's dangerous and what's not. For instance, bacteria, viruses and cancer cells are dangerous and should be attacked, while the food you eat is not (usually) dangerous and should not be attacked. The challenge for the immune system is that each bacteria or virus needs its own special response. Think about each bacteria and virus as an individual lock, and the immune system needs a key for each lock.

Now, the immune system doesn't know IN ADVANCE what infections a person is going to get, so it tries to have as many keys on hand as possible, so that if a virus or bacterium comes along, it will have the proper response to fight them. But the immune system's goal of having as many keys as possible must be tempered by two things: first, it must not have any keys that will allow it to make an immune response against the body's own cells (to avoid autoimmunity), nor should it have any keys that will allow it to make an immune response against harmless things like food that you eat or dust or pollen. So during the development of the cells that hold the keys for the immune system, cells that hold keys for self cells or harmless things are thrown out. Unfortunately, this process is not perfect, and not all the keys that should be thrown out will be thrown out. When this happens, autoimmunity and allergies occur. So this is one way allergy to penicillin might happen: the immune system fails to throw out the key to penicillin, resulting in an inappropriate immune response.

A second way allergy to penicillin can happen is through what is known as molecular mimicry. What this means is that the lock for penicillin and the lock for something harmful that the body has seen before, like strep bacteria, are very similar, and so the key that helped the body's immune system deal with the strep also fits the penicillin lock. When this person is later given penicillin, that person's body mistakes the penicillin for strep, and causes an inappropriate response.

Alternatives for penicillin are other antibiotics such as erythromycin, or zithromax, but what antibiotic is appropriate for a particular infection should be determined by a doctor. I hope I have answered your question, and if you have any further questions, feel free to e-mail me at: ingrid_dodge@student.hms.harvard.edu. To learn more about penicillin, visit:
http://falcon.cc.ukans.edu/~jbrown/penicillin.html, or http://www.discoveryhealth.com and search penicillin. Take care! Ingrid References: 1) Padovan, E. "T Cell Response in Penicillin Allergy." Clinical and Experimental Allergy. 28 Supplement 4: 33-36, September 1998. 2) Weltzien HU, E Padovan. "Molecular Features of Penicillin Allergy." Journal of Investigative Dermatology. 110(3): 203-206, March 1998. 3) Kuby J. Immunology, 3rd Edition. New York: WH Freeman and Co., 1997. 4) Paul, WE. Fundamental Immunology, 4th Edition. New York: Lippincott- Raven, 1999.