Where allergies are known and have been diagnosed, learning how to navigate daily life can be difficult. When an allergy or intolerance is unknown, it can be exhausting and frustrating dealing with the symptoms that arise. The difference between an allergy and an intolerance is the involvement of the immune system. An allergy results in your immune system reacting to a normally harmful food or substance as if it is a toxin and, in some cases, this can be life-threatening. An intolerance is a bad reaction to a food or substance that does not involve the immune system and although it can be unconformable, it is not life-threatening Apart from the well-known allergic reaction symptoms, there are some other signs that can indicate an allergy or intolerance may be present.