Related Clues:

• Show surprise, perhaps
• Flinch or blink, say
• Smile, e.g.
• Block a punch, e.g.
• Get revenge
• Acknowledge a stimulus
• Pay back in kind
• Not just sit there
• Hit back, say
• Undergo chemical change
• Not keep a poker face
• Duck, say
• Gasp, say
• Fight or flee
• Get a rash, perhaps