I believe "reaction medium" would be the term normally used in the context of chemical processes - for example:
https://link.springer.com/article/10.1007/s10098-003-0242-7

However, AFAIK there is no such thing as the "body's reaction medium" (this sounds a little like pseudo-science).

So you may instead want to use something like "the body's natural reactive state" or "the body's natural reactive capability."

http://amilighttherapy.com/professional-systems.html
Circulation supports the natural reactive capability of the immune system and strengthens the body's own regulatory mechanisms to activate its self-healing powers, thereby providing support for the healing of wounds and injuries and all regenerative processes in the body.

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Note added at 4 days (2017-06-07 05:57:29 GMT) Post-grading
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You are welcome.