How can you be allergic to peanuts but not walnuts?

Allergies are specific to certain proteins found in foods. Peanuts and walnuts contain different protein structures, and the immune system may react to the proteins in peanuts while tolerating those in walnuts. Genetic and environmental factors also play a role in determining individual sensitivities. Therefore, it is possible for someone to be allergic to one type of nut while being able to consume another without issue.