Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Food allergies are like other allergies. They occur when the immune system reacts to harmless substances as though they were harmful. Almost ten percent of children have food allergies. Some of the foods most likely to cause allergies are shown below ( Figure below ). Answer: immune system
The reaction of which body system causes food allergies?