Hydration Biology: How Water Regulates Detoxification & Immunity” is an in-depth scientific exploration of how water governs cellular metabolism, toxin clearance, immune signaling, and disease reversal. Integrating biochemical theory with thousands of clinical observations from Save Medha Foundation, the paper demonstrates that hydration is not merely a lifestyle habit but a core biological requirement essential for sustaining life and restoring health. The study reveals that dehydration impairs every major detoxification system—liver, kidneys, lymphatic flow, and skin—leading to toxin accumulation, inflammation, lymph node enlargement, tumor formation, high WBC reactions, and immune system hyperactivity. Water is shown to be the primary solvent that enables nutrient transport, enzyme activation, oxygen delivery, toxin dissolution, and the mobility of immune cells. Through Natural Immunotherapy (NIT), hydration becomes a therapeutic tool: warm water therapy, micronutrient support, detox boosters, and lymph-cleansing protocols significantly improve creatinine, urea, lymph node swelling, tumor softness, inflammation markers, and overall immunity. The paper positions hydration biology as the foundational element of cellular intelligence and a prerequisite for metabolic correction, immune stability, and disease recovery. By establishing a comprehensive biochemical model of how water regulates detoxification and immunity, this research proposes hydration as the first and most essential step in reversing chronic diseases—and an indispensable pillar of the Natural Immunotherapy framework developed at Save Medha Foundation with clinical support from Dantura Botanics.