Given below are two statements :
Statement $I:$Autoimmune disorder is a condition where body defense mechanism recognizes its own cells as foreign bodies.
Statement $II:$Rheumatoid arthritis is a condition where body does not attack self cells.
In the light of the above statements, choose the most appropriate answer from the options given below:
$R.B.C.$ in adult are formed
An 'antigen' is
An antibody is
Give scientific reasons : Autoimmune diseases are different from immune deficiency diseases.