In chemistry, a chemical reaction is a change in which one or more chemical elements, molecules, species, or compounds (the reactants) form new molecules, species, or compounds (the products). [1] In short, a chemical reaction is a process that results in the interconversion of chemical species. [2]

Human chemical reactions
A chemical reaction between two or more people (human molecules) is called a "human chemical reaction". [3]

References
1. Daintith, John. (2004). Oxford Dictionary of Chemistry, (pg. 125). New York: Oxford University Press.
2. Chemical reaction - IUPAC Goldbook.
3. Thims, Libb. (2007). Human Chemistry (Volume One), (preview), (pg. 42). Morrisville, NC: LuLu.