What is Contact Allergic Dermatitis and How Does it Affect the Immune System?

Contact allergic dermatitis, as the name suggests, refers to an allergic inflammation of skin in response to contact with an allergen: in other words, a reaction to something we touch. The reaction is a type 4 hypersensitivity: a slow and somewhat cum­bersome process. It starts when allergen penetrates the skin. Small molecules penetrate further than larger ones, which is why we so readily become allergic to smaller molecules, such as nickel. Immune cells, normally resident in skin, pick up the allergen and travel with it to the lymph glands. This journey takes about twenty-four hours. These special cells then present the allergen to the immune system and ask it for judgment: do we 'tolerate' this allergen, or 'reject' it?

Certain people are genetically predisposed to reject more than they tolerate, and these are the individuals who develop contact allergic dermatitis. Their immune system sends a hostile message back to the site of original contact. Before long, they end up with a full-blown allergic inflammation in the skin. Furthermore, now that the immune system has been alerted to this allergen, it will post cells all over the body which have been programmed specifically to watch out for it. That's why a con­tact allergy, once established, will eventually occur wherever the allergen is placed, and not just at the site of original contact. Christine, for example, developed an itchy rash on her left wrist. Some time later she developed a similar rash on her ear lobes, and finally, by the time she came to the Allergy Clinic, she had a rash just below her navel! Christine was allergic to nickel, for she had dermatitis wherever she had contact with this allergen, namely in her watch strap, earrings and belt.

Come back now for a moment to this notion of 'original con­tact'. The allergen may be fixed in one spot, but immune cells are not! Thus, an allergy to the nickel in a watch strap will affect the underlying skin at the wrist, but the allergic reaction may spread up the arm as far as the shoulder! The nickel, as I say, is not moving, but the immune cells are. Now consider such migration of allergy away from the sites of watch strap, earrings, necklace, buckle and bra straps! You can imagine how easily the pattern of 'original contact' is quickly lost in this scenario. Never fear, allergy (patch) tests will reveal the truth.