Possible Clues:

• Line in general not normally something to get a reaction
• Protein making everyone turn green
• Go across the Channel with information that may produce adverse reaction
• Sneeze-causing substance
• Work in a particular key can cause an abnormal reaction
• Wheat, say, somewhat smaller, generally
• Pollen, often
• Right to enter claim with name which provokes a reaction
• Any substance that causes a hypersensitive reaction
• Dust or pollen, e.g.