The degree of sensitivity of allergies varies widely. For example there are people who only have a problem after consuming a large scoop of peanut butter but there are also those who will end up in the hospital from trace amounts that you'd have difficulty spotting with the naked eye.

I dated a woman with celiac sprue (which I guess was extreme.. her mother had to have a bowel resection due to celiac related issues) and she had sudden anaphylaxis at a restaurant that required the use of an epi-pen and an ambulance.

The reaction was caused by the micro-brewery that had opened next door and all the wheat dust in the ventilation system.