Final answer:
Systemic anaphylaxis is caused by the presence of allergen in the circulation. Epinephrine can be used to counter the reaction.
Step-by-step explanation:
Systemic anaphylaxis is caused by the presence of allergen in the circulation. Anaphylaxis is a rapid and severe allergic reaction that can be life-threatening. When an allergen enters the bloodstream, it triggers a massive release of histamines which can cause a drop in blood pressure, swelling of the tongue and throat, and difficulty breathing. This systemic reaction can be countered with the use of epinephrine, which constricts blood vessels and relaxes smooth muscles to improve breathing and reduce swelling.