Active Spanish-Catalan bilinguals with confirmed Alzheimer's disease demonstrated superior performance on attention, executive function, language, and visuospatial tests compared to passive bilinguals at identical disease stages. The 567-participant study revealed that active bilingualism provides dual protection: cognitive resilience despite existing pathology, and biological resistance through altered cerebrospinal fluid and plasma markers indicating reduced amyloid burden and neuroinflammation. This represents a significant advance in understanding how bilingualism mechanistically protects against neurodegeneration. The findings suggest that actively using multiple languages throughout life may literally rewire the brain's response to Alzheimer's pathology, offering both functional compensation and biological disease modification. This could inform prevention strategies emphasizing language learning and maintenance. However, the observational design cannot establish causation, and cultural confounding factors may influence results. As an unreviewed preprint, these promising findings require peer review and replication across diverse populations before clinical recommendations can be made. The research nevertheless provides compelling evidence that bilingualism operates through multiple neuroprotective pathways.