Possible Clues:

• Reason to say "Bless you!"
• React to ragweed
• Prelude to Bless you!
• Not blow hot, blow cold
• By the sound of it, one knows where this comes from
• Go a-tishoo!
• What a blow one gets from the beginning of 13 across
• Blow that for when it's cold
• What a blow that it's not cold like 27 across
• Minor bodily explosion