allergic reactions are caused by para-phenylenediamine (PPD) used for henna shine, for those who do not know henna (temporary paint used for tattooing).
It is treated by topical and sometimes oral corticosteroids, antibiotics given in the case of pustular form, as well as antihistamines for itching.
Oh, my goodness!! That might make me think twice before doing it, especially if one were prone to allergies.