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Background: Alzheimer’s disease (AD) is a multifactorial neurodegenerative disorder characterized by beta-amyloid plaques, neurofibrillary tangles, and progressive cognitive decline. Recent evidence has highlighted the role of blood–brain barrier (BBB) dysfunction in the early stages of AD pathology. Objective: To explore the histological structure and physiological function of the blood–brain barrier, and to identify the shared pathological mechanisms between BBB disruption and Alzheimer’s disease progression. Methods: This narrative review was conducted through a comprehensive search of peer-reviewed literature from 1997 to 2024, using databases such as PubMedh, Elsevier, Scopus, and Google Scholar. Results: Multiple histological and cellular components—including endothelial cells, pericytes, astrocytes, and tight junctions—contribute to BBB integrity. The breakdown of this barrier in AD is associated with chronic inflammation, oxidative stress, vascular injury, pericyte degeneration, astrocyte polarity loss, and dysfunction of nutrient transport systems like GLUT1. These alterations promote neuroinflammation, beta-amyloid accumulation, and progressive neuronal damage. Conclusion: BBB dysfunction is not merely a consequence of AD but may act as an early and active driver of its pathogenesis. Understanding the mechanisms of BBB breakdown can lead to early diagnostic markers and novel therapeutic strategies aimed at preserving or restoring barrier integrity in Alzheimer’s disease.