Peanut and tree nut allergy is most common in infants and young children, but may appear for the first time in adults. Peanut allergy often receives greater attention because it is common (3% of infants), exposure is hard to avoid and in some cases even trace amounts can trigger symptoms. Around 20% of cases resolve, and while severity may occasionally lessen with age, around 20% of cases can become worse with time.
Food proteins trigger food allergy
The substances triggering allergic reactions to food are the proteins in the food. If a person is allergic to one protein present in one food only (such as peanut or egg) then an allergic reaction can only occur if they eat that one particular food. Some individuals may be allergic to more than one protein in more than one food and so may be allergic to several foods.
What does allergy to a nut or seed really mean?
The meaning of the terms nuts, seeds and legumes is confusing, particularly for allergic patients (or their parents) trying to decide what foods to avoid. For example, the term "legume" is often used to describe peanut as well as other plants like peas, chickpeas and soy yet this group of plants also includes wattles and the black bean tree of Queensland.
The term "tree nuts" also has limited meaning, as the foods that we consume from these plants come from a wide variety of different botanical families such as Rosaceae (almonds), Anacardiaceae (cashews), Proteaceae (macadamia nuts) or Lecythidaceae (Brazil nuts).
When we think of seeds, we often think of small seeds like sesame seed, sunflower seed, poppy seed or pumpkin seed. In fact, coconut (including the husk and inner white flesh that we eat) is also a seed, albeit a very large one! Many of the foods that we consider to be "nuts" are in fact part of a seed or its food source, often with the outer fruit or coating removed.
Therefore, while we often use the terms "tree nuts" or "seeds" to describe some foods, these categories are rarely useful for predicting allergy to foods of similar appearance or taste. With few exceptions (e.g. most people allergic to cashew are also allergic to pistachio), it is not possible to reliably predict the likelihood of allergy to seed or nut-like food without allergy testing to that particular food.
Peanuts are legumes, not nuts
Peanuts are legumes, like peas, lentils and chickpeas (and also diverse other plants like wattles and the black bean tree of Queensland). The proteins in peanut are very different to those in tree nuts (such as almonds, Brazil nuts, cashews, hazelnut, macadamia nuts, pecans, pistachios or walnuts). Therefore someone allergic to peanut is not automatically going to be allergic to tree nuts.
Peanut allergy is becoming more common
A recent Australian study has shown that peanut allergy affects 3% of children under 1 year of age. There are many theories as to why this increase has occurred, but more research is required.
Cross reactivity is difficult to understand and harder to predict
Cross reactivity means that a similar protein is present in a range of different foods. If the same protein is present in several foods, then that person may have allergic reactions to any food containing that protein. Examples of cross reactivity include people allergic to similar proteins present in hen's egg and duck eggs; or cow's milk and goat's milk; or cashew nut and pistachio nut.
It is sometimes difficult to predict whether a person will be allergic to one unique protein allergen present in one food only, or several similar cross reactive proteins present in multiple foods, simply based on whether foods have a similar appearance. Therefore it is not possible to reliably predict the likelihood of allergy to seed or nut like food without allergy testing to that particular food.