Reactions can be taken after you have Passed.
After you have Passed in the Action Phase the only Actions you may take are Reactions. To take a Reaction, move an Influence Disc from your Influence Track to the space on the Reaction Track of your Summary Tile that corresponds to the chosen Reaction. The possible Reactions are:
Upgrade: Execute one Upgrade Activation.
Build: Execute one Build Activation.
Move: Execute one Move Activation.
Techs granting extra Activations do not apply to Reactions!