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. Some of the symptoms are similar, such as wheezing, but shortness of breath, the most common asthma symptom, is not a common allergy symptom. The treatments for these conditions are different too. A lot of people who suffer from allergies have asthma too, and some of them aren't even aware of it.