8 year-old that weighs 55lbs comes in for prophylaxis . . . This patient has been in for a prophylaxis, multiple times,

[answerhappygod]

8 year-old that weighs 55lbs comes in for prophylaxis . . . This
patient has been in for a prophylaxis, multiple times, in the past.
At this current visit, the patient’s mother mentions that after the
previous visit, the patient had redness and itching around his
mouth, and a runny nose that went away after a day or so, but a
call to the patient’s pediatrician had him taking Benadryl after
the appointment. You put on your gloves and start the prophylaxis
treatment. The patient immediately develops redness across his face
and chest, difficulty breathing with wheezing, and becomes light
headed and confused, then loses consciousness.
• What happened?
• What do you do?