There is probably no medical device that has been studied more intensively in terms of the potential to stimulate an immune response than breast implants. (Allergy is an immune system response.) Since both saline and silicone gel implants have a silicone shell, the question is a very important one. A few years ago, there were a couple of reports in the medical literature of allergy-type reactions to implants, but it relates more to impurities and not the silicone itself, and in any case this is a handful of cases out of millions of women with implants. That would pretty much be the definition of hypoallergenic! As manufacturing standards have improved over the past 15-20 years, these types of cases should be even more rare now. Silicone is probably the most "biocompatible" synthetic material known, meaning that it generates very little response and so the body accepts it very well. Of all of the substances we are exposed to in a typical day, silicone is among the most benign.