As we have already discussed, people may be allergic to a number of different substances, referred to as allergens. People who are allergic to one substance are often allergic to others.

Some of the most common allergens include:
- Venom from insect bites and stings, especially those of bees, wasps, hornets, and yellow jackets
- Foods, including nuts, shellfish, crustaceans, peanuts, milk, eggs and chocolate
- Plants, including contact with poison ivy, poison oak, and pollen from ragweed and grasses
- Medications, including penicillin and other antibiotics, aspirin, seizure medications, muscle relaxants and over-the-counter remedies
- And finally, other things such as dust, latex, glue, soaps and make-up