Alzheimer's is a type of dementia, dementia being a neuro-degenerative condition resulting in the loss of neurones and therefore decreed brain function.
Neurons (nerve cells) produce an extracellular protein on the the outside surface of the cell membrane. This protein known as APP (Amyloid Precursor Protein) is broken down as part of the cells normal behaviour by the enzyme below.
This enzyme cleaves (breaks) the protein into two components, one remains embedded inside the cell membrane and other falls into the extracellular space and is removed.
In Alzhiemer's, there is different enzyme behaviour.
These two enzymes cleave the protein into 3 different sections, one section remaining intracellular , the remaining extracellular section of the protein is cleaved into 2 sub-units. These one part of these sub units goes on to form BETA-AMYLOID protein which is sticky. It coagulates with other BETA-AMYLOID protein to form plaques.
These plaques are neurodegenerative (toxic to neurones) causing the loss of grey matter, enlarged ventricles and therefore loss of neurological function.