Our body has its own defence force called “AntiBodies”. Whenever there is an attack from foreign bacteria/virus/fungus, called “Antigens”, the body produces “Antibodies” to fight them.
So, in normal cases, Antibodies are our heroes, but sometimes, they become our enemy.
Instead of killing outsiders, Antibodies start attacking our own cells because they identify our own cells as foreign bodies.
Apart from overactive defence, overgrowth of cells in certain body parts is also a reason for an autoimmune disorder.