What is the definition or description of: autoimmune?

Self reacting: The immune system developed to tell our own, normal cells (self) from foreign and abnormal cells (non-self). This lets the immune system eliminate viruses, bacteria, fungi and cancer cells from our body without harming normal cells. Sometimes the immune system fails to tell self from non-self and it attacks normal cells, for example in joints causing rheumatoid arthritis.

Attacking self: An autoimmune process occur when your immune system attacks cells in your body that it shouldn't attack. The immune system is supposed to attack foreign invaders such as infections but if the immune system does not discriminate it may attack cells or parts of cells that are normally found internally and are not foreign invaders.

Self-reaction: Body reacts against itself is the definition, but it is simply a way of categorizing.