The similar sounding terms cross-contact and cross-reactivity are a source of confusion in the food allergy community. Although both refer to safety precautions, these concepts have important differences that matter in day-to-day safety planning for people with food allergies.
Cross-contact describes a minute amount of allergen that gets into a food where it isn’t intended to be an ingredient. Risk of cross-contact is the reason for “may contain” warnings or “processed in the same factory as” allergy warnings on food products. Cross-contact can also happen in kitchens when proper precautions aren’t taken to keep allergens separate from safe foods.
Cross-reactivity occurs because two different allergenic foods have a similarity at a molecular level. When cross-reactivity happens, the immune system develops a reaction to both foods. Tree nuts are a common example: it’s quite common for someone who has an allergy to one tree nut to develop allergies to other tree nuts, so generally tree nuts are classed together as a group for allergy precaution purposes.
Cross-contact happens only through physical contact but it can happen with any allergen; cross-reactivity can happen without physical contact but it tends to happen in specific known patterns.
Let’s take an example and say someone tests positive for pecans and for walnuts. The allergist advises them to avoid tree nuts. They wonder if it would be OK to eat almonds if they confirm that neither pecans nor walnuts are processed at the same factory. Information from the manufacturer about other nuts handled at that facility could provide information about cross-contact but would not ensure safety from cross-reactivity. Basically, the risk is that a protein in almonds has a similar enough molecular shape that it might interact with the antibodies that already respond to pecans and to walnuts.
If that person really loves almonds and wants to try them, then the safe route would be to contact the physician’s office and ask about challenge testing. Challenge testing provides a chance to see whether a person’s immune system reacts to a risky food while a team of trained healthcare professionals are on hand to respond in case a reaction does occur. Among several types of challenge test, the gold standard is double blind placebo controlled food challenge — which uses precautions that prevent either a patient’s fears or a physician’s assumptions from affecting the outcome.
Some foods contain more than one allergenic molecule. In instances where this happens the cross-reactivity can be different according to which allergen causes the reaction. To use almonds as an example once again, another type of almond allergy is unrelated to tree nuts. This other protein in almonds is chemically similar to birch pollen and has a different set of cross-reactivities, most of which are in the Rosaceae botanical family. The Rosaceae family includes apples, peaches, pears, nectarines, plums, raspberries, blackberries, and several other common fruits. This latter type of reaction is a manifestation of Oral Allergy Syndrome which receives less attention than tree nut allergy because fewer than 2% of OAS cases escalate to anaphylaxis.
People who develop the tree nut version of almond allergy have reason to worry about other tree nuts but need not worry about cross-reactivity with Rosaceae fruits. The reverse is true about people who develop the OAS version of almond allergy. This distinction might not be obvious when an allergy first develops, which is one reason to see an allergist for testing.
Some references use the term cross-contamination interchangeably with cross-contact while other sources make a distinction to apply cross-contamination only to foodborne pathogens such as e. Coli, which are distinct from allergies. Experts occasionally disagree on terminology and this is one point of contention. If you see “cross-contamination” in the context of a discussion about allergies, then be aware that it could mean “cross-contact” but it never means “cross-reactive.”
To conclude with a few other practical notes, warnings such as “may contain” or “processed in the same factory as” are voluntary in the United States. Absence of such a warning does not imply safety. The FDA only regulates whether cross-contact statements are truthful. So a guarantee such as “processed in a factory free from [allergen]” must be true.
A user friendly set of guidelines for avoiding cross-contact in ordinary settings is available here.