Alzheimer's disease. Computer artwork of the early and late stages of Alzheimer's disease. Neurons (nerve cells) are blue, an astrocyte cell is green and a microglial cell is purple. Alzheimer's is a neurodegenerative disease and a common cause of dementia in the elderly. It is caused by the formation of protein plaques (brown) in the brain, which kill surrounding neurons. Microglial cells are phagocytic cells that digest dead nerve cells and pathogens. In Alzheimer's the cells are activated by the plaques but do not attack them. However, they remain activated and secrete substances that kill surrounding neurones. Astrocytes are nerve support cells that are able to clear the plaques. It is thought that defects in astrocytes allow the formation of plaques. The cause of Alzheimer's is not known and there is no cure.