An autoimmune disease is a condition in which your immune system mistakenly attacks your body.
The immune system normally guards against germs like bacteria and viruses. When it senses these foreign invaders, it sends out an army of fighter cells to attack them.
Normally, the immune system can tell the difference between foreign cells and your own cells.
In an autoimmune disease, the immune system mistakes part of your body — like your joints or skin — as foreign. Thus, it loses its ability to differentiate between the self and non-self antigens. In response to these antigens, it releases proteins called antibodies that attack healthy cells.

Ex- Rheumatoid arthritis, Psoriasis, vitiligo

Alzheimer's disease is not an autoimmune disease. It is a progressive disorder caused by a combination of genetic, lifestyle and environmental factors that affect the brain over time. People suffering from Alzheimer's disease have a difficulty in remembering day to day activities, eventually leading to dementia.