Final answer:
Type I hypersensitivity reactions are mediated by IgE on basophils and mast cells.
Step-by-step explanation:
Type I hypersensitivity reactions, also known as immediate hypersensitivity reactions or allergies, are mediated by IgE on basophils and mast cells. When an individual is sensitized to an allergen, IgE antibodies bind to the surface of basophils and mast cells. Upon subsequent exposure to the same allergen, the allergen binds to the IgE antibodies, triggering the release of chemical mediators such as histamine, leading to an allergic reaction.