Not all dermatitis is a reaction to an allergen. Irritant contact dermatitis happens when you come in contact with a substance that damages the skin [sources: Dillon, Mayo Clinic]. The irritants remove oils and moisture from your epidermis. Without the natural protection normally provided by these, the irritants can penetrate more deeply and trigger the immune system, causing inflammation. The severity of irritant dermatitis depends on several factors, including how much irritant came in contact with the skin, the chemical makeup and strength of the irritant, how long and how often the skin is exposed, and whether or not the skin has a tendency to damage easily [source: CCOHS].
Like irritant contact dermatitis, allergic contact dermatitis is an external skin reaction (most allergic responses are internal reactions). However, allergic contact dermatitis is an overreaction in the body's immune system to an allergen, not an irritant. Say you're allergic to latex. The first time you come in contact with latex, you won't have an allergic reaction; your body uses that first contact to learn the allergen is harmful. The second time you touch latex, your body "remembers" that you're allergic and produces warning signs. The body's natural histamine, or a compound that is released by the body's immune system, rushes to the contact site and warns you -- with a rash and itchy skin [source: Dillon]. The body's response to the harmful substance, not the substance itself, produces the rash. Some people may have stronger allergic reactions than others due to hypersensitivity to certain substances [source: Dillon].
Everyone's immune system reacts differently to allergens. It's possible to start developing an allergic reaction to a substance that you have been exposed to for many years. Once the substance becomes an allergen to you, however, it will cause an allergic reaction for the rest of your life. Sometimes a substance may not be a problem until you are in the sunlight. A few products that can cause this type of "photocontact" dermatitis are sunscreens, creams and lotions containing sulfa medication and perfumes [source: Mayo Clinic].
The symptoms of contact dermatitis can be almost as varied as the substances that cause it. To see what some of these symptoms are, continue to the next page.