If it's an allergy, your immune system is involved. After you touch something, it mistakenly thinks your body is under attack. It springs into action, making antibodies to fight the invader. A chain of events causes a release of chemicals, including histamine. That's what causes the allergic reaction -- in this case, an itchy rash. It's called allergic contact dermatitis.
Common triggers for contact dermatitis are:
Poison ivy, poison oak, and poison sumac
Hair dyes or straighteners
Nickel, a metal found in jewelry and belt buckles
Leather (specifically, chemicals used in tanning leather)
Citrus fruit, especially the peel
Fragrances in soaps, shampoos, lotions, perfumes, and cosmetics
Some medications you put on your skin
Usually, you won't get a rash the first time your skin touches something you're allergic to. But that touch sensitizes your skin, and you could have a reaction the next time. If you got a rash on first contact, chances are you've touched that trigger before and just didn't know it.
Some rashes look like an allergic reaction but really aren't, because your immune system wasn't involved.
Instead, you touched something that took away the surface oils shielding your skin. The longer that thing stayed on your skin, the worse the reaction. It's called irritant contact dermatitis.
If you have eczema, you're more likely to get this kind of rash.
What's Causing Your Rash?
Many of the symptoms can be the same. In both cases, your skin may blister, or you may get a raised red rash. Your skin will itch and maybe burn.
When something is irritating or damaging your skin, you'll probably see a rash right away. With an allergy, it may be a day or two before the rash shows up.
Symptoms of a contact allergy are usually right around where you touched the thing you're allergic to.
Irritant contact dermatitis (skin damage) tends to be more painful than itchy.