Possible Clues:

• Abnormal reaction as to, say, pollen
• Abnormal sensitivity to a food
• Unfavourable reaction to, say, pollen
• It might be largely an overreaction
• Something to sneeze about?
• Hypersensitivity to a substance
• It needs the whole of the grey mixture to make one so sensitive
• Hypersensitive reaction to certain substances
• Unfavourable reaction to pollen, certain foods etc
• Producer of Hay Fever put the lead almost at the back of the cheap seats