The triggers that cause allergy are called allergens, and they are usually protein molecules. They may enter the body through skin contact, through inhalation, by injection, or by ingestion.
Latex is an example of an allergen that may enter the body through skin contact or inhalation. Bee or wasp venom and drugs like anaesthetic agents typically enter the body through injection.
Foods like nuts, milk, eggs or fish and some medications can cause allergic reactions through ingestion. Inhaled allergens include pollens, moulds, house dust and animal dander.
For people with asthma, there are factors that are not allergens that can cause or worsen symptoms; these include exercise, emotion, exposure to cigarette smoke, seasonal changes and respiratory tract infections.