The prevailing reductionist paradigm in medicine partitions the organism into independent organ systems and autoimmune conditions into unrelated “diseases,” each with its own biomarkers and patented treatments. This paper advances an integrative, systems-level hypothesis: the brain operates as the sovereign coordinator of the whole body, and the amyloid precursor protein (APP) — particularly its β-secretase cleavage products — constitutes the brain’s universal, first-line molecular signal in response to any threat to its integrity. We propose that stagnant, oxygen-depleted blood (venous stasis, microvascular hypoxia) is a prime initiator of this signal, triggering platelet degranulation and local APP release, which then feeds back to the brain to activate an innate immune “catastrophe” cascade. When the threat becomes chronic, the brain escalates its defense, orchestrating a targeted autoimmune assault on the tissues it perceives as compromised. We illustrate this model with myasthenia gravis (MG), where thymic venous congestion leads to aberrant APP processing, complement deposition at the neuromuscular junction, and the generation of anti-acetylcholine receptor antibodies. Building on this framework, we propose a liquid biopsy panel (plasma Aβ40/42 ratio, sAPPβ, BACE1 activity, platelet APP isoform ratio, and Aβ oligomers) that can be implemented immediately with existing ultra-sensitive assay platforms. This panel constitutes a Universal Brain Threat Index (UBTI), a single, non‑invasive score reflecting the brain’s perception of bodily distress. Crucially, the UBTI is not merely a diagnostic for active disease; it is designed as a periodic screening tool—analogous to intraocular pressure measurement in glaucoma—to detect silent threats (sleep apnea, chronic venous insufficiency, pre-autoimmune states, vascular cognitive risk) years before clinical manifestation. Testable predictions are outlined, including the normalization of UBTI after thymectomy in MG prior to antibody decline, and its correlation with intermittent hypoxia in sleep apnea. The hypothesis reframes autoimmunity as the brain’s desperate, adaptive survival response, and calls for a return to a coherent, whole-body medicine where the brain’s own distress call is monitored annually as a vital sign of kingdom-wide integrity. Keywords: Amyloid Precursor Protein; Liquid Biopsy; Autoimmune Disease; Myasthenia Gravis; Hypoxia; Brain-Defense; Universal Brain Threat Index; Preventive Medicine; Early Detection; Periodic Screening; Glaucoma Analogy; Pre-Autoimmune State; Blood Stasis; Coherent Medicine