Anaphylaxis, or a life threatening allergic reaction, is a time critical medical emergency which requires quick management and intervention. Many people know of someone, or have come across a person who may be "anaphylactic" (severely allergic) to some allergen, whether it be a wasp sting, nuts, of certain fruit. When the body is exposed to that specific allergen, it's autoimmune response goes into an 'overkill' mode which then unfortunately causes major problems for the body's other systems. A common issue is inflammation, swelling around the face, mouth, throat, and neck can quickly occlude the airway and cause the individual to suffocate. Another issue is the displacement of fluids in the body. The individuals blood pressure can significantly drop and cause hypo-perfusion to major organs, causing damage and death.
During the initial faze when an anaphylactic individual accidentally comes into contact with an allergen, they may panic and alert you to the fact that they have eaten something they're allergic to, or that their throat might be starting to swell up. During this initial realization, an ambulance needs to be called immediately. Time is of the essence now, and emergency help will be required as soon as possible. If the individual was stung by an insect, use a credit card to scrape away the stinger, don't try to pick at it or squeeze it out as this may squeeze further poison from the stinger into the skin making the situation worse. An individual who suffers from anaphylaxis will often carry an epipen with them. This is an injection that will reverse the body's effects in trying to fight the allergen. Make sure that they take it as soon as possible to start the reversal process. The individual may even carry piriton or chlorphenamine, other anti-allergen drugs, if they have them, remind them about them as they could take them before the swelling gets too bad so they can't swallow. If the individual hasn't got these items on them and the ambulance hasn't arrived yet, see if other people around you have them. Ensure that the individual takes the items themselves and don't force it upon them, there may be unknown reasons why they cannot take the items which you may not be aware of.
If the individual collapses, place them in the recovery position and make sure someone is waiting to flag down the ambulance. The 999 telephone operator will be able to provide further advice on how long the ambulance will be, and what more you can do until the ambulance arrives.