The rarest type of allergic reaction is called anaphylactic shock. This is a severe reaction that can affect many organs at once. Symptoms include rapid decrease in blood pressure, rash or hives, difficulty breathing, abdominal pain, swollen tongue or throat, diarrhea, fainting and sometimes death. Essentially, an allergic reaction occurs when the body sends a signal to the immune system that a harmful substance is present. These IgE antibodies react with allergens to release histamines.
People can develop allergies at any time in their lives.
You could spend your childhood playing in the woods and never develop a poison ivy rash, but then be exposed to it as an adult and have a severe reaction. Asthma is a condition that is made suddenly worse by triggers. Some triggers are also allergens. This may make it sound like asthma and allergy are the same, but that's not the case. As asthma attack is a different process than an allergic reaction.