Kids with severe allergies (such as those to food, medicine, or insect venom) can be at risk for a sudden, potentially life-threatening allergic reaction called anaphylaxis. Anaphylaxis can happen just seconds after being exposed to an allergen or not until a few hours later (if the reaction is from a food).

So doctors will want anyone diagnosed with a life-threatening allergy to carry an epinephrine auto-injector in case of an emergency. Epinephrine works quickly against serious allergy symptoms; for example, it reduces swelling and raises low blood pressure.

Airborne Allergy Symptoms
Airborne allergens can cause something known as allergic rhinitis, which usually develops by 10 years of age, reaches its peak in the teens or early twenties, and often disappears between the ages of 40 and 60.

Symptoms can include:

sneezing
itchy nose and/or throat
stuffy nose
coughing
When symptoms also include itchy, watery, and/or red eyes, this is called allergic conjunctivitis. (Dark circles that sometimes show up around the eyes are called allergic "shiners.")

Food, Medicines, or Insect Allergy Symptoms
wheezing
trouble breathing
coughing
hoarseness
throat tightness
stomachache
vomiting
diarrhea
itchy, watery, or swollen eyes
hives
swelling
a drop in blood pressure, causing lightheadedness or loss of consciousness
Allergic reactions can vary. Sometimes, a person can have a mild reaction that affects only one body system, like hives on the skin. Other times, the reaction can be more serious and involve more than one part of the body. A mild reaction in the past does not mean that future reactions will be mild.

-kidshealth.org