Cross-reactivity occurs when the immune system of a person allergic to one food (e.g. peanut) reacts to a similar protein in another food (e.g. chick pea). This usually occurs when the immune system  reacts to foods or plants in the same botanical family.

Scientific information on cross-reactivity can be found in the Informall database:

Click on individual foods, then on “view clinical data”, and then scroll down to “IgE cross-reactivity and Polysensitisation”.

For the average reader, the following are a few examples of cross-reactivity that can occur. Remember, however, that they are not to be taken as general rules. For example, one person with peanut allergy may react to other legumes such as peas or lentils, while another person will not. All cases require a proper diagnosis, preferably at an allergy clinic.

Latex-fruit syndrome

Some of the proteins present in latex also exist in a similar form in certain fruits.  If a person is allergic to the protein in latex that is also present in banana, for example, they may have a cross-reaction if they eat a banana. Apart from banana, the foods that commonly cause problems for people with latex allergy are avocado, kiwi, chestnut and occasionally walnut.  Anyone reporting a reaction to any of the above foods may also be latex-allergic, and vice versa.

Pollen-fruit syndrome

Pollen-food syndrome – also known as oral allergy syndrome – describes an increasingly common sensitivity to certain raw fruits and vegetables, including apple, carrot, celery, melon, and cherry. The condition is explained by cross-reactivity to certain pollens (particularly birch pollen). Reactions to the same foods when cooked are less likely. Symptoms include redness, swelling and itching of lips, tongue and inside mouth. Occasionally there is itchy swelling of the throat. Most cases are mild.

Where allergic reactions to fruit or vegetables are reported, it is useful to know if the patient also suffers hay fever. This would support a diagnosis of pollen-food syndrome.

Reactions to legumes

U.S. research showed that five per cent of selected population of legume reactive children had symptoms with multiple legumes. For example, a small proportion of people with peanut allergy react to green peas. A study of life threatening and fatal allergic reactions in Sweden pointed very strongly to soya as the cause of a number of these reactions in people known to be allergic to peanuts. Conclusion: A person with who reacts to one legume (such as peanuts, soybeans, lentils, chick peas and beans) might react to some others in the same group.

Peanuts and fenugreek

A Norwegian allergy register recorded 15 peanut-allergic patients reacting to curry, which often contains fenugreek, a legume. Doctors studied cross-reactivity using serum from a peanut-allergic patient. The patient’s serum was positive for fenugreek and two grains on the patient’s lip caused an immediate reaction. Allergy to fenugreek may be considered a possibility when someone with peanut allergy reacts to curry.

Sunflower seeds and Jerusalem artichoke

A woman suffered anaphylaxis after eating a packet of seeds purchased from a health shop. On seeing an allergist, she tested positive to sunflower seeds. One day the woman made a soup using Jerusalem artichoke. Soon after eating it she suffered symptoms of allergy. She saw an allergist, who performed a skin prick test for Jerusalem artichoke, and the result was strongly positive. She found out to her surprise that Jerusalem artichoke is a relative of the sunflower.

Almonds and apricot kernels

At a UK allergy clinic, 12 nut-allergic children were tested for allergy to apricot kernel. Two out of the 12 had positive tests: one boy with peanut allergy, the other with almond allergy. Although oral challenge testing would have to be done to support the findings, there is clear evidence of cross-reactivity. Apricot and almond belong in the same family, along with peach, plum and cherry. Any food company planning to use crushed apricot kernels as a substitute for almonds needs to consider the possibility that reactions may occur in people with nut allergy.