Welcome and thank you for your question.
A person cannot be allergic to her own bodily fluids.
Chronic vaginal itching
can be due to many causes like infections, especially yeast infection and allergies. She needs blood tests to look for diabetes, thyroid, kidney or liver problems, etc. A Skin patch
test as well as Patch Testing for contact dermatitis is also necessary.
Sometimes, a biopsy of the vulval skin may also be necessary.
I am sure that her doctor has tested her for these. If not, please ask for these tests.
A common cause for chronic vaginal itching when no cause can be found is hypersensitivity of the nerves of the skin. This hypersensitivity can be triggered off by an infection or allergy and can remain even after the initial infection or allergy has cleared up.
It is difficult to test for hypersensitivity. But your daughter can be prescribed a drug called gabapentin (Neurontin) which can help.
Anti-depressants like Amitriptylline are also very effective in chronic vaginal itching.
I hope this helps.