What is cross reactivity? Many chemicals are very strong and can be extremely sensitising to the skin and body. For example, chemicals in hair dyes such as PPD para-phenylenediamine, TDA toluene-2,5-diamine and other aromatic amines are different chemicals, but they have similar structures that can trigger skin irritation. Fragrances are often the culprit of cross reactivity becuase they contain thousands of different chemicals rolled into one ingredient on the product label.
In food, cross-reactivity occurs when protein is present. "If the same protein is present in several foods, then that person may have allergic reactions to any food containing that protein. Examples of cross-reactivity include people allergic to similar proteins present in hen's egg and duck eggs; or cow's milk and goat's milk; or cashew nut and pistachio nut." - Allergy.org
WHAT DO THE EXPERTS SAY?
"Many of the approved ingredients of oxidative hair dyes are strong or even extreme contact allergens; for instance, p-Phenylenediamine (PPD, 1,4-diaminobenzene, CAS no. 106-50-3), toluene-2,5-diamine (TDA, 1,4-diamino-2-methylbenzene, CAS no. 95-70-5; synonym p-toluenediamine, PTD) and p-aminophenol (CAS no. 123-30-8) are three important precursors or intermediates associated with hair dye-related allergic contact dermatitis." And, "Cross-reactivity has been described as a contact allergic reaction to a molecularly similar chemical to which the individual has not yet been exposed. The immune system is not capable of differentiating between the sensitizing chemical and the almost similar new chemical" - Mdpi.com Department of Dermatology, University Medical Center Groningen, University of Groningen, 9700 RB Groningen, The Netherlands.