|"The only known vaccine against the 'Gray Death.' Unfortunately, it is quickly metabolized by the body making its effects temporary at best."|
|— Description of Vial of Ambrosia, Deus Ex|
Ambrosia is described as a "non-organic compound containing eutactic components". The Ambrosia vaccine consist of ROM modules which attach themselves to the Gray Death nanites, thus acting as a "hardware-lock". Like the Gray Death, it can only be manufactured using a Universal Constructor.
The beginning of Deus Ex involves the UNATCO agent JC Denton recapturing several containers of Ambrosia which were stolen by the NSF in an attempt to return it to the people. The NSF attempted to ship a sample of Ambrosia to Hong Kong, with the hope that Tracer Tong would be able to reverse-engineer the vaccine and produce it independently.
MJ12 own and control every functioning Universal Constructor on the planet. Considering that a UC is needed to manufacture Ambrosia, they alone decide how much Ambrosia gets manufactured and who it gets distributed to. In this way, MJ12 use the Gray Death and by extension, Ambrosia, as a means to control and manipulate others into doing their bidding.
Vaccines normally contain a component of a pathogen (virus, bacteria etc.), which is subsequently identified, neutralized, and remembered by the infected individuals' own immune system, granting them long-term immunity. However, it seems that Ambrosia does not work this way. Instead, it latches onto the pathogen (a Gray Death nanite in this case) and prevents it from functioning.
This is very similar to the way antibodies work in animal immune systems. Antibodies attach to a specific part of a pathogen (called an antigen), and either mark it for destruction by other cells in the body, or directly disrupt its ability to function.
Furthermore, real-world vaccines normally give long-term immunization of at least a few years. In Deus Ex, this supposed vaccine offers immunity for a mere 48 hours. Also, vaccines do not offer a "cure" of any sort to someone who has already been infected by a pathogen. This contradicts the ability of Ambrosia to temporarily subside the symptoms of the Gray Death, suggesting that it is not a vaccine at all.
- The name "Ambrosia" is one of many symbolic names taken from Greek mythology. In Greek mythology, Ambrosia was the food of the gods, which gave them immortality.