http://www.ck12.org Chapter 1. Human Biology
Allergies
Anallergyoccurs when the immune system attacks a harmless substance that enters the body from the outside.
A substance that causes an allergy is called an allergen. It is the immune system, not the allergen, that causes the
symptoms of an allergy.
Did you ever hear of hay fever? It’s not really a fever at all. It’s an allergy to plant pollens. People with this type of
allergy have symptoms such as watery eyes, sneezing, and a runny nose. A common cause of hay fever is the pollen
of ragweed (Figure1.116).
FIGURE 1.116
Ragweed is a common roadside weed
found throughout the United States. Many
people are allergic to its pollen.Many people are allergic to poison ivy (Figure1.117). Skin contact with poison ivy leads to an itchy rash in people
who are allergic to the plant.
As you have read, some people are allergic to certain foods. Nuts and shellfish are common causes of food allergies.
Other common causes of allergies include:
- Drugs such as penicillin.