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 anaphylactic reactions are unlikely given the typical quantity of any ingredient in a hair product and the relatively small area of exposure, but they are not entirely unheard of.