Call 911 immediately if your baby is having trouble breathing or passes out. Lay him down with his feet elevated to reduce the risk of shock. Try to keep him calm by talking to him and by remaining calm yourself.
Don't give your baby an antihistamine if he's younger than 6 months. Even if he's older, don't give him an antihistamine if he's having any trouble breathing or swallowing, because he may choke on it.
When the paramedics arrive, they'll probably treat your baby on the spot with an injection of epinephrine that will stop the reaction within minutes. (Epinephrine makes the heart beat more strongly, relaxes the muscles in the airway, reduces swelling, and improves tone in the blood vessels to increase blood flow to vital areas like the heart and brain.)
The paramedics will take your baby to the hospital, where he'll be examined and watched for delayed reactions. The doctors at the hospital can help you determine what caused the problem. You'll want to follow up with your baby's doctor, who will probably refer you to a pediatric allergist.