It's not necessarily the case that someone with peanut allergy will have a tree nut allergy. Some people with tree nut allergies can eat peanuts and vice versa.
Although we call the peanut a "nut," technically it is not a nut. It is a legume, or a legume fruit, that grows in a pod. Some other well-known legumes are peas, beans, lentils and soy. Legumes grow on a plant that is the ground. Nuts are different because they grow on trees and have a hard shell, such as cashews, pine nuts, pecans, pistachios, walnuts, and even coconut.
Why are people with peanut allergies told to stay away from tree nuts?
One of the reasons why those who have a peanut allergy are told to avoid tree nuts is because many people have both allergies. According to the American Academy of Allergy Asthma & Immunology, 25% to 50% of those allergic to peanuts also have allergies to tree nuts. Rather than having an allergy test for each kind of tree nut, it is usually more convenient to avoid all peanuts and nuts.
This is especially true for young children who might have an easier time avoiding all nuts than trying to identify types of nuts. It's also the case that different types of tree nuts and peanuts are processed with the same equipment which means that there is a high risk of cross contamination.
It's not so easy to avoid peanuts and nuts
Having food allergies means that you can no longer safely eat any processed food without carefully checking every ingredient on the label. Peanuts and other nuts are often used in restaurant foods too, especially in Indian, Thai, Mexican, Chinese, and other restaurants. If you have a severe reaction to peanuts or nuts, it is too risky to eat in a restaurant for some people. They need to carry medication with them that can be injected to prevent serious consequences from consuming a food that may have been ground or chopped in a food processor that previously chopped peanuts.
Peanuts and tree nuts are ground and used in many foods
Once a peanut or tree nut allergy has been confirmed, it is necessary to find out which foods have these hidden in them. People without allergies don't usually pay much attention to food ingredients, but if something in a candy bar or bakery item can make you very sick, you will want to check it. If you have a mild allergy to these, you may be able to trust the ingredients on a package if it does not list peanuts or tree nuts.
Food labels have improved, but not all can be completely trusted
More food manufacturers are beginning to tell consumers on their labels if a food was produced in a plant where peanut products are used. However, even if machinery is thoroughly cleaned after peanut butter cookie dough was in it, there could still be small amounts in the machinery that could affect a person with a severe allergy. Most allergists advise their patients not to eat these foods and to skip them altogether.
Peanuts are a surprising ingredient in many foods
Did you know that peanut butter is used in making egg rolls? It is also used in some sauces and salad dressings that you would probably never suspect. Cereals, candy bars, ice cream, cookies, crackers, chili, and other foods often have peanuts in them. You can make your own peanut free baked goods, candies, and cereal at home or buy them at a website that you trust that sells peanut free foods.