The ((nowiki))<nowiki))</nowiki>X-Antibody''' (X抗体 X-Koutai?)</nowiki> is a plot element from ''[[Digital Monster X-Evolution]]'', ''[[Digimon Chronicle]]'', ''[[Digimon D-Cyber]]'', and the related [[Digimon virtual pet#Digimon Pendulum X|virtual pets]] and [[Digimon card games|card games]]. It is a vaccine that makes Digimon immune to the [[X-Program]], and most Digimon that are introduced to it also become more powerful than their normal counterparts. Some Digimon (for example, [[Dorumon]]) are unaffected by the X-Antibody because they actually possess it naturally.

When the Digital World became so overpopulated, the super computer that governs the Digital World, [[Yggdrasil]], wasn't able to handle it anymore. His solution was to wipe out the vast majority of the Digimon with the X Virus, and choose a very small percentage to be moved to a new Digital World, and destroy the old one. Those who were not chosen but still survived and moved to the new world had a rare gift known as the X-Antibody. This antibody, when activated, changed their appearance and strength.

Those who have the X-Antibody are generally distinguished by the X in their name and sometimes an X placed on their body somewhere or a gem (example: Dorumon's on his forehead, WarGreymon X's on the front portion of his front horn). There are few with the X-Antibody that isn't distinguished by an X after their name because there is no other version of them in existence.

The X Digimon fled to a new Digital World which was split into three Terminals (the three terminals are named after the [[wikipedia:Norns|Norns]] from Norse Mythology) consisting of '''Urd''', a past plain which is a volcanic wasteland, inhabited by dinosaur and dragon Digimon, '''Verdandi''' is the present which is a world of lush greenery and is home to beast, bird, plant and other nature Digimon, and '''Skuld''' is the future, a high-tech city where machine and insect Digimon dwell. Yggdrasil sent the [[Royal Knights]] to eliminate the X Digimon.

While X Digimon appear in ''Digimon D-Cyber'', Yggdrasil and the X-Antibody are never mentioned, as all Digimon are already X Digimon. Furthermore, while the Royal Knights who appear in X-Evolution and Chronicle are main characters, they are never shown to have any kind of mission to exterminate the X Digimon.

Also, in America, Digimon with the X-Antibody are prominently featured in the card series '''Digimon: Operation X''' Certain Digimon have the X Antibody, some Digimon are vulnerable to these Digimon, and another group of Digimon can easily destroy Digimon with the X-Antibody. Modify cards can also add, destroy, alter, or get power from the X Antibody.
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The {{nihongo|<nowiki>'''X-Program'''|Xプログラム|X-Puroguramu}}</nowiki> is a termination program initiated by [[Yggdrasil]] in the 8th digimon movie [[Digital Monster X-Evolution]] to eliminate 98% of digimon as a result of the [[wikipedia:exponential growth|exponential growth]] of digimon causing the Digital World to reach maximum capacity. When the X-Program was executed by Yggdrasil, 98% of the total digimon population was eliminated. The remaining 2% were transferred to the [[NEW Digital World]] where the X-Program could not reach them. However, a minority number of digimon strong enough to resist the effects of the X-Program were able to survive by changing their DigiCores. The digimon were reborn in a form that could take in the X-Program and eventually it came to a point where a program that opposed the X-Program was installed into the [[DigiCore]] - the [[X-Antibody]].
===X-Antibody Indicator===
The {{nihongo|<nowiki>'''X-Antibody Indicator'''|X抗体インジケーター|X-Koutai Indikeetaa}}</nowiki>, also referred to as the {{nihongo|<nowiki>'''XAI System'''|XAIシステム|Sai Shisutemu}}</nowiki> is a system which displays the status of the [[X-Antibody|X-Antibodies]]. If the XAI system is activated, via the [[Pendulum X]], and it detects the X-Antibody present, it will begin to alter the digimon itself, resulting in an X-Antibody Digimon.