Natural latex is derived from trees and made into gloves, condoms, and other latex products. The latex rubber that makes up these products is therefore, not surprisingly, often contaminated with other proteins from the trees it is harvested from. It turns out that most people with latex allergies are not allergic to the latex itself, but to one or more of the plant proteins that contaminate it.
Since polyisoprene is created in a clean laboratory environment, it does not suffer from the same contamination problems as natural latex. It has, by and large, the same physical properties as latex without the natural byproducts that tend to cause a bit more biological havoc. That's why polyisoprene condoms are so similar to latex condoms. Polyurethane, on the other hand, is a different type of polymer entirely, which is why those condoms tend to be baggier and less stretchy.
If you have latex allergies and are looking for products that can make your sex life safer, it may be worth some experimentation with one of the polyisoprene condoms on the market. They may be a better option than polyurethane for some people because of their lower price. Many men also prefer the way they fit. It may seem strange that scientists have created a latex-based latex allergy condom, but it actually makes a lot of sense.
Alenius H, Makinen-Kiljunen S, Turjanmaa K, Palosuo T, Reunala T. "Allergen and protein content of latex gloves." Ann Allergy 1994;73:315-20.
Palosuo T, Mäkinen-Kiljunen S, Alenius H, Reunala T, Yip E, Turjanmaa K. "Measurement of natural rubber latex allergen levels in medical gloves by allergen-specific IgE-ELISA inhibition, RAST inhibition, and skin prick test." Allergy. 1998 Jan;53(1):59-67