a.

1. 反应性的；易反应的
2. 反作用的；反动的

adj.

1. showing a response to a stimulus.
2. acting in response to a situation rather than creating or controlling it.
3. having a tendency to react chemically.
4. Physiology  showing an immune response to a specific antigen.
5. (of an illness) caused by a reaction to something.
6. Physics  relating to reactance.