Define/Explain: Pathogens and Auto-immunity.

(N/A) In higher vertebrates,the memory-based acquired immunity is based on the ability to differentiate between self and non-self (e.g.,pathogens). Although the exact mechanism is not fully understood,it is based on two corollaries:
$(i)$ Higher vertebrates can distinguish between foreign molecules and foreign organisms. Most of experimental immunology is based on this aspect.
$(ii)$ Sometimes,due to genetic or other unknown reasons,the body attacks its own cells,resulting in damage to the body. This is known as an $Auto-immune$ disease.