The X-Antibody (X抗体 X-Koutai) is a vaccine that makes Digimon immune to Program X. Digimon that are introduced to the X-Antibody undergo a change in physical appearance (e.g. crystals emerging on various parts of a Digimon's body) and usually become more powerful than their previous, non X-Antibody counterparts. Some Digimon already have the X-Antibody present within them.
When the Digital World reached its maximum capacity, the host computer of the Digital World, Yggdrasill, was unable to manage. Its solution was to eliminate the vast majority of the Digimon, with the Program X and select a very small percentage (1%) to be moved to a NEW Digital World, while destroying the old one. Some of those who were not chosen, still survived and moved to the new world by altering their Digicore to resist the Program X, forming the X-Antibody as a resulting consequence.
The X Digimon fled to the NEW Digital World which was split into three terminals consisting of Ulud, a past plain which is a volcanic wasteland, inhabited by Dinosaur type and draconic Digimon, Versandi, the "present" region which is a world of lush greenery and is home to beast, bird, plant and other nature Digimon and Skuld, the "future" region, a high-tech city where machine and insect Digimon inhabit. Yggdrasill sent the Royal Knights to eliminate the X Digimon.
Digimon World X is based on the history of the X-Antibody. In the game, the X-Antibody can be used to increase the powers of Digimon as well as objects.