React  v. 1 (often foll. By to) respond to a stimulus; change or behave differently due to some influence (reacted badly to the news). 2 (often foll. By against) respond with repulsion to; tend in a reverse or contrary direction. 3 (foll. By with) (of a substance or particle) be the cause of chemical activity or interaction with another (nitrous oxide reacts with the metal). 4 (foll. By with)  cause (a substance) to react with another.