Artificial red dye doesn't usually cause true allergic symptoms, but it can cause allergy-like symptoms in people who are sensitive to it. However, naturally occurring red food coloring can cause a dangerous allergic reaction on rare occasions. A sensitivity to dyes like Red-40 is most likely to result in hives or swelling. However, an allergy to insect-based dyes like carmine and cochineal can result in anaphylaxis, while annatto coloring from achiote trees can produce an allergic response that involves symptoms in the skin, respiratory system and digestive system.
The difference between intolerance to artificial additives and an allergy to the natural colorings is that artificial colorings don't involve the immune system, whereas the natural colorings do. While true allergic response are rare, anaphylaxis is a serious and potentially life-threatening condition that can be recognized by a drop in blood pressure, constricted airways, a rapid pulse, dizziness and even loss of consciousness.
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