The THC in cannabis PREVENTS the deposit of amyloid plaque in the brain. “THC blocks an enzyme called acetylcholinesterase, which speeds the formation of amyloid plaque in the brains of people with Alzheimer's disease. The Alzheimer's drugs Aricept and Cognex work by blocking acetylcholinesterase. When tested at double the concentration of THC, Aricept blocked plaque formation only 22% as well as THC, and Cognex blocked plaque formation only 7% as well as THC.” From “Marijuana May Slow Alzheimer's” at WebMD.

And furthermore, THC activates the CB2 receptors which can do this- “The activation of cannabinoid CB2 receptors stimulates in situ and in vitro beta-amyloid removal by human macrophages” and  “Activation of the CB(2) receptor system reverses amyloid-induced memory deficiency”    (both at PubMed).

(FYI- THC activates the anti-inflammatory CB2 receptors;  in situ means on the site, in this case in a brain; in vitro means in a test tube;  macrophages are your “clean up crew” cells.)

And one last quote- "individuals who smoked marijuana in the ‘60s and ‘70s, who are now entering their 60s and 70s, are not getting Alzheimer’s at the rate they should be." – Dr. Gary Wenk, PhD, professor of neuroscience, immunology and medical genetics at Ohio State University.