What is an allergy?
It is a magic spell.
No.
Allergy is an overreaction of our immune system to usually harmless substances.
Like peanuts, milk, pollen, etc.
These substances are called allergens.
Our immune system has white blood cells or lymphocytes which protect us from foreign invaders.
Such as bacteria, viruses, parasites, etc.
These invaders are called antigens.
When a lymphocyte detects an antigen, it produces antibodies.
Antibodies destroy antigens and thus, protect us.
Now, if an allergen such as pollen is inhaled by an allergic person.
The lymphocytes of the allergic person for unknown reasons think that the allergen is an antigen.
Hence, lymphocytes overreact and produce antibodies which start attacking the allergen.
Besides this, as a protective response, some immune cells also trigger production of excess mucus.
Resulting in running nose.
They even give rise to itching throat and watery eyes, thus causing an allergy.