An allergy is a reaction by the immune system to a harmless substance known as an allergen. Ragweed pollen is just one of many weed pollens that can act as an allergen.
When someone with an allergy comes in contact with their allergen, the immune system jumps into action to defend the body from what it believes is a foreign invader. This triggers the body to release antibodies and other chemicals including histamine.
Histamine triggers typical allergy symptoms including itchy or puffy eyes, runny or stuffy nose, sneezing, and sore throat. For people with asthma, contact with an allergen can also trigger an asthma attack which can make breathing difficult.
Ragweed is a weed that grows in most parts of the United States except in upper New England and the southern tip of Florida. Seventeen species of ragweed can be found in the United States, including giant ragweed.