State one reason why a person could be allergic to peanuts, but not be allergic walnuts

Peanuts are legumes and, unlike tree nuts, grow underground. Although peanuts are not tree nuts, people with a peanut allergy have a similar allergic reaction as those with a tree nut allergy. If you have one tree nut allergy, it's highly likely that you're allergic to other tree nuts as well.