Can someone explain to me "idiosyncratic allergic reaction"?

Idiosyncratic Rx. An idiosyncratic allergic reaction is an unexpected adverse reaction, typically due to a drug or food, not involving the immune system in the same way that one might reaction to a pet, pollen or a food allergy like peanut. For example, hives from Aspirin or x-ray contrast are due to their chemical properties in the body, not allergic antibodies. Allergy skin or blood tests are not helpful.