Anaphylaxis is an extreme and severe allergic reaction that can occur as a result of exposure to any substance which causes an allergic reaction (an allergen). The reaction can occur within minutes of exposure, but sometimes after hours. Anaphylactic reactions can occur as a result of vaccination or administration of any drug.
If a patient experiences an anaphylactic reaction, then treatment must be administered immediately, using an Adrenaline / Epinephrine 1 in 1000 injection (e.g. Epipen, Jext).