Final answer:
The type I hypersensitivity reaction that occurs in a previously sensitized host to pollen is called an allergy. It involves the production of IgE antibodies and activation of mast cells, leading to the release of inflammatory mediators like histamine.
Step-by-step explanation:
The type I hypersensitivity reaction that occurs in a previously sensitized host to a foreign substance, such as pollen, is called an allergy. When a person is exposed to an allergen, such as pollen, their immune system produces IgE antibodies that bind to mast cells. When the person is subsequently exposed to the same allergen, the allergen binds to the IgE antibodies on the mast cells, triggering the release of inflammatory mediators like histamine, which leads to the symptoms of an allergic reaction.