When you put jewelry in your body (or even wear it, as with a watch), there is some transfer of material from the jewelry to your body. Usually this is simply absorbed and disposed of or sloughed off as waste along with dead skin and is inconsequential. However, some individuals will display varying degrees of sensitivity to different metals - a metal allergy.
Today, a sensitivity to nickel is the most common, and said to be on the rise along with reactions to latex.
For a person with a metal allergy, piercings performed using that metal (or using an alloy containing that metal) will either take longer to heal, or, less commonly, react aggressively with the jewelry. Symptoms include redness and swelling around the piercing and dry cracked skin on the sore. You can differentiate it from other problems because it will look like the skin is trying to "pull away" from the jewelry, and the redness will extend around the jewelry in all directions more so than simply along the fistula.
This problem almost exclusively happens with stainless steel jewelry due to the nickel content, and the easiest solution is to use titanium or niobium. Switching to gold will usually not help because gold in body jewelry is an typically made of an alloy which tends to contain nickel and other metals prone to reactions. Passivated jewelry is also an option depending on the level of allergy.