When you have allergies, certain things don’t “agree” with your body. These things may be foods, animal hairs, plant pollen, chemicals, or almost anything else, and may enter the body when you eat or inhale.
When they do, you feel uncomfortable or even sick. Although millions of people have this problem, scientists still aren’t sure what causes them.
It seems that when something that you’re allergic to enters your body, certain cells in the body begin to produce chemicals called antibodies, which normally help fight disease and infection. But with an allergy, the antibodies combine with the foreign substance and cause the body to produce other chemicals, called histamines.
These histamines disturb the mucous membranes, blood, lungs, and other organs, producing an allergic reaction. That’s why many of the drugs that are used by people with allergies are called antihistamines.