Allergy science decodes how the immune system reacts to harmless stimuli and why hypersensitivity manifests across the skin, airways and gut. This first volume presents the immunologic foundations of allergic disease, clinical phenotypes (eczema, allergic rhinitis, asthma, food allergy), and standard diagnostic approaches. It explains testing methods--skin tests, specific IgE, component-resolved diagnostics--and interprets their strengths and limitations. Practical chapters cover acute management of allergic reactions and long-term strategies including immunotherapy and avoidance counseling. Flowcharts, action plans and patient-education scripts make this a hands-on resource for clinicians and allied health professionals. The text targets learners building a robust, mechanism-based approach to allergy care.