 [ Symptoms ] what is cross reactivity as it relates to food allergies [ Disease ] cross reactivity is the occurrence of allergic reactions to foods that are chemically or otherwise related to foods known to cause allergy in an individual if someone has a life threatening reaction to a certain food the doctor will counsel that patient to avoid related foods which also might induce the same reaction for example if a person has a history of a severe allergy to shrimp he or she can also possibly be allergic to crab lobster and crayfish