Tezepelumab in Allergic Rhinitis and Asthma Study (TEZARS)

The goal of this clinical trial is to learn about how well tezepelumab (pronounced TEZ e PEL ue mab), a Health Canada-approved drug for severe asthma, works in participants with coexisting allergic asthma and allergic rhinitis (hayfever). The main questions it aims to answer are: * How well does the study drug work to reduce nasal symptoms following exposure to an allergen after 6 doses, and after 12 doses? * Does the study drug reduce chemicals found in the nose known to be involved in the allergic rhinitis response? Both allergic and non-allergic participants will be enrolled in the study. Non-allergic participants will: * Not be receiving the study drug. * Be exposed to allergen via the Nasal Allergen Challenge, which involves the administration of allergen extract directly to the nose. * Complete nasal symptom and quality of life questionnaires * Have nasal fluid and blood samples collected at various time points up to 24 hours following allergen exposure. * Visit the clinic 3 separate times: * At a screening visit to determine their eligibility to participate in the study * At the baseline Nasal Allergen Challenge visit * At the 24-hour post-Nasal Allergen Challenge follow-up visit Allergic participants will: * Receive one dose of tezepelumab every 1 month for 12 months. Tezepelumab will be administered as an injection into the fatty layer just beneath the skin on the stomach, arm, or thigh. * Be exposed to allergen via the Nasal Allergen Challenge, which involves the administration of allergen extract directly to the nose. * Complete nasal symptom and quality of life questionnaires * Have nasal fluid and blood samples collected at various time points up to 24 hours following allergen exposure. * Visit the clinic 17 separate times: * At a screening visit to determine their eligibility to participate in the study * At the baseline, 6-month, and 12-month Nasal Allergen Challenge visits * At each 24-hour post-Nasal Allergen Challenge follow-up visit * For each dose of the study drug The investigator will compare changes in nasal symptoms and allergic chemicals measured from nasal fluid and blood samples between non-allergic participants and allergic participants at baseline and at 6- and 12-months following the use of the study drug.