Similarly there is a difference between allergies affecting various parts of the body. A "Gut Allergy" for example will probably not show up in a skin prick test, or a RAST blood test but will present as predominantly gut symptoms and WILL involve the immune system.
The Government has produced a useful page on children's allergies here and BUPA have here .
True allergies can be tested for, by the traditional skin prick test or by blood tests called RAST tests. This is a "radio-immune assay test and is described