When it comes to tree nuts from the same tree family, such as pecans and walnuts, if you have an allergy to one, you are likely to have an allergy to the other. Likewise, if you can tolerate one, you can likely tolerate the other. While pecans are less likely to trigger an allergic reaction, walnuts are the second most common nut allergy in the U.S. Nut allergies are developed when the body's immune system is oversensitive to particular nut proteins, and it is possible to have allergies to one nut and not to others. However, if you have or suspect you have a nut allergy, it is best to avoid consuming walnuts and pecans without discussing or taking the proper allergy tests with your physician.