Sensitization is the word used to describe an allergic reaction
that develops after repeat exposure to an allergen. Often, the
first time a body is exposed to a substance it does not seem to
know what to do with it and basically just observes it. However,
in subsequent exposures, it apparently recognizes the substance,
and if it deems the material a dangerous intruder, it mounts an
attack (immune response) against it. Occasionally, sensitization
occurs after one small exposure, but more often it occurs after
a single high dose or large surface area exposure, or after
prolonged, repeat exposures over either small or large surface
areas. This is why products such as hair dyes and relaxers
recommend that you patch test a small area of skin every single
time you plan to use the product. Symptoms of sensitization to
ingredients in hair care products can include development of a
painful, pimply rash, scaly, itchy atopic dermatitis, contact
dermatitis, eczema, inflammation and itchy, flaky scalp.
Systemic and/or ana