Cross-reactivity in allergic reactions occurs when the proteins in one substance are similar to the proteins found in another. Because peanuts and soy are both legumes, there is concern that peanut-sensitive individuals will also react to soy. However, clinical studies indicate that cross-reactivity between peanut and soy is a relatively infrequent occurrence. According to the University of Manchester, “Most soy allergic individuals are tolerant of peanut …This does not mean that absolutely no patients exist that demonstrate cross-reactive allergy to multiple legumes, but it is rare.”

Allergen cross-reactivities occur when IgE antibodies, originally raised against a specific allergen, bind to identical or highly similar surface areas of another related allergen. These cross-reactivities are largely determined by secondary and tertiary structural similarities between allergens. The general thinking is that cross-reactivity requires more than 70% sequence identity, while proteins that share less than 50% sequence identity are rarely cross-reactive. This also implies that cross-reactive proteins have a similar three-dimensional fold and belong to the same protein family.1 As somewhat of a side note, there is also evidence indicating that IgE cross-reactivity exists between unrelated allergens, although that type of cross-reactivity is less common.2