Choroid plexus tissues are intraventricular structures composed of villi covered by a single layer of ciliated, cuboid epithelium. The plexuses secrete cerebrospinal fluid (CSF), synthesize numerous molecules, carry nutrients from the blood to CSF, reabsorb brain metabolism by-products and participate in brain immunosurveillance. During ageing, atrophy of epithelium occurs along with thickening of basement membranes. Enzymatic activities of epithelial cells decrease significantly. CSF secretion decreases as much as 50%. These modifications are concurrent with subnormal brain activity. In Alzheimer's disease, epithelial atrophy, thickening of basement membrane and stroma fibrosis are even more prominent. Ig and C1q deposition along the basement membrane can be frequently detected, suggesting immunological processes. Synthesis, secretory, and transportation functions are significantly altered resulting in decreased CSF turnover, reduced beta-amyloid clearance, and increased glycation phenomena as well as oxidative stress. Such modifications may favour fibrillary transformation of beta-amyloid protein and tau protein polymerisation.