Food families have similar proteins and this can lead to cross-reactive allergy. That having been said, not all families have the same degree of cross-reactivity.
Peanut allergy is often severe but, luckily, has one of the lowest levels of cross-reactivity with other legumes. There is only a 5 percent risk of cross-reactivity for peanut with beans, peas and soybeans.
Cow-milk allergy is the highest (at 90 percent), with other mammals and milks from sheep and goat. Yet, people with cow-milk allergy almost never react to mare’s milk, donkey’s milk or dolphin’s milk.
Shellfish cross-reactivity is high at 75 percent. Thus if an individual is allergic to shrimp, he or she has a three in four chance of also being allergic to lobster, crabs or crawfish. This high degree of cross-reactivity is not true for non-crustacean shellfish such as clams and oysters.
Fish cross-reactivity is roughly 50 percent.
Tree nut allergy is about 35 percent cross-reactive across the board. However, certain nuts seem more closely linked. Pistachio is very similar to cashew, walnut to pecan and almond to hazelnut. Nut allergy does not translate to seed allergy such as sesame seed, but sesame, poppy and sunflower tend to be cross-reactive with one another.
On a practical level, if an individual with a specific food allergy has already tolerated other foods in the same family, that person should continue to be safe in eating these family-related foods. Still, foods in the same family that have not yet been tried should be considered suspect, and the individual may want to undergo allergy testing to determine the safety of the particular food.