Alzheimer's disease does not have one single cause. It results from a complex interaction of biological processes that lead to abnormal protein accumulation, synaptic dysfunction, neuronal injury and progressive neurodegeneration. The defining pathological features include amyloid-beta plaques and abnormal tau aggregates, but these changes occur within a broader biological environment involving neuroinflammation, vascular dysfunction, oxidative stress, impaired cellular clearance and other processes. Age is the strongest risk factor for the common late-onset form, but ageing alone does not explain why one person develops Alzheimer's disease and another does not. Genetic susceptibility, cardiovascular and metabolic health, life-course exposures and biological variation all contribute to risk.
Amyloid-beta is one of the central pathological features of Alzheimer's disease. It originates from processing of amyloid precursor protein, a normal membrane-associated protein found in neurons and other cells. Different enzymes can cleave amyloid precursor protein through different pathways, and one pathway generates amyloid-beta peptides. In Alzheimer's disease, particular amyloid-beta species can aggregate into soluble assemblies and eventually extracellular plaques.
Amyloid accumulation is associated with synaptic dysfunction and other downstream biological effects, although the relationship between amyloid deposition and neuronal injury is complex. The amyloid cascade hypothesis has strongly influenced Alzheimer's research, but contemporary models recognise that amyloid is one component of a multifactorial disease process rather than a complete explanation for every clinical feature.
Tau pathology is another central mechanism. Tau normally supports the structure and function of neuronal microtubules, which are involved in intracellular transport. In Alzheimer's disease, tau undergoes abnormal biochemical changes and forms intracellular aggregates known as neurofibrillary tangles. Abnormal tau is strongly associated with neuronal dysfunction and loss, and the spatial distribution of tau pathology correlates more closely with clinical severity than amyloid burden alone in many studies. Tau pathology tends to emerge and spread through anatomically connected neural systems, eventually affecting widespread cortical networks. This helps explain why Alzheimer's disease progresses from relatively circumscribed cognitive deficits to more extensive impairments involving multiple domains.
Neuroinflammation contributes to Alzheimer's disease biology. Microglia, the brain's resident immune cells, normally remove cellular debris and participate in maintaining neural tissue. In Alzheimer's disease, microglial responses to amyloid, damaged neurons and other signals can become chronically altered. Astrocytes and other immune-related mechanisms also participate. Inflammatory signalling may contribute to synaptic dysfunction and neuronal damage, while impaired immune clearance can potentially allow pathological material to accumulate. Neuroinflammation is therefore neither simply a protective response nor an independent cause; it interacts with amyloid, tau, vascular processes and neuronal injury. Research continues to investigate which inflammatory pathways are harmful, protective or dependent on disease stage.
Genetic factors influence susceptibility. The APOE gene has three common alleles, with APOE ε4 being the most important common genetic risk factor for late-onset Alzheimer's disease. Carrying one or two APOE ε4 copies increases statistical risk, but many carriers never develop Alzheimer's disease and many people with Alzheimer's disease do not carry APOE ε4. By contrast, rare pathogenic variants in APP, PSEN1 or PSEN2 can cause autosomal-dominant early-onset Alzheimer's disease, in which affected families may show disease across successive generations. These inherited forms account for a small minority of Alzheimer's cases. Genetic information therefore ranges from strong causal mutations in rare families to common risk variants that alter susceptibility without determining individual outcomes.
Vascular and metabolic factors can influence Alzheimer's disease risk and progression. Hypertension, diabetes, smoking, obesity, high cholesterol, physical inactivity and other cardiovascular risk factors are associated with increased risk of dementia, including Alzheimer's disease, although an association with dementia risk does not prove that a particular factor directly causes Alzheimer's pathology in an individual. Cerebrovascular disease can reduce brain resilience and may interact with amyloid and tau pathology.
Blood-brain barrier dysfunction, impaired cerebral blood flow and vascular inflammation are active areas of research. The frequent coexistence of vascular pathology and Alzheimer's pathology is particularly important because cognitive impairment in later life often reflects several interacting disease processes rather than a single isolated pathology.
Ageing is the dominant risk factor for sporadic late-onset Alzheimer's disease, but the biological reason is multifaceted. With age, cellular repair mechanisms, protein-clearance systems, vascular regulation, mitochondrial function and immune regulation can change. Synaptic and neuronal resilience may also decline. These changes can increase vulnerability to pathological processes, but normal ageing and Alzheimer's disease are not identical. Some older people accumulate amyloid or tau pathology with limited clinical impairment, while others develop substantial neurodegeneration and dementia. Cognitive reserve, education, lifelong intellectual activity and other factors may influence the relationship between pathological burden and clinical symptoms, although these factors do not make a person immune to Alzheimer's disease.
The exact causal sequence remains an area of scientific investigation. Evidence supports important roles for amyloid and tau, but researchers continue to examine how these proteins interact with genetic susceptibility, inflammation, vascular dysfunction, lipid metabolism, cellular clearance, synaptic failure and other mechanisms. This complexity explains why a single intervention may affect one biological feature without completely stopping disease progression. It also explains why Alzheimer's disease should not be attributed simply to "forgetting", ordinary ageing, one lifestyle factor or one gene. The most accurate current understanding is that Alzheimer's disease arises from interacting biological processes that progressively damage neural networks, with age and genetic susceptibility being major determinants of risk and vascular, metabolic and other factors potentially modifying vulnerability and disease expression.
