Final answer:
Eosinophils are the leukocytes that increase during allergic reactions and are responsible for releasing inflammatory substances that cause allergy symptoms alongside basophils and mast cells.
Step-by-step explanation:
The type of leukocyte known to increase during allergic reactions is the eosinophil. Allergic reactions are typically associated with a type I hypersensitivity, during which mast cells and basophils release inflammatory molecules such as histamines upon stimulation. The role of eosinophils is particularly crucial as they target parasitic infections and are also involved in mediating allergic responses. In such reactions, eosinophils along with basophils are responsible for releasing inflammatory substances that can cause allergy symptoms. Antihistamines work by blocking the action of histamine, thereby reducing allergy symptoms. Given the effectiveness of antihistamines in treating the symptoms, it indicates that a patient with severe, persistent allergy symptoms might have had increased activity of eosinophils before treatment.