Memory based acquired immunity evolved in higher vertebrates based on the ability to differentiate foreign organisms (pathogens) from self cells.

While we still do not understand the basis of this, two corollaries of this ability have to be understood.

Higher vertebrates can distinguish between foreign molecules as well as foreign organism. Most of the experimental immunology deals with this aspect.

Sometimes due to genetic and other unknown reasons, the body attacks self cells. This results in damage of the body and is called as Auto Immune disease.

Rheumatoid arthritis which affects many people in our society is an auto-immune disease.