It is a reaction from our immune system to something we got in contact with that the immune system did not recognize as "safe".
The substance that causes such a reaction is called an "allergen".
Allergens can be found all around us.
The primary reaction that will happen is the immune system trying to destroy that "invader" (allergen), and for that, it uses a significant amount of the immune mediators.
The outcome is an allergic reaction that can come in all sorts of presentations and severity.
Saying that not all allergens we get in contact with will make our immune system react.
Some are relatively harmless, depending on each individual's immune system and, often, prior exposure to that substance.
What are the most common allergens in children?
How does it affect children:
The outcome of the allergies depends on the food the child is allergic to.
Milk, soy, egg and wheat - are the most common foods involved in non-IgE mediated food allergy.
Dr Costa is a Consultant Paediatrician and fellow of the Royal College of Paediatrics and Child Health.