What is auto-immune disease?Give an example.

Hint: Auto immune disease is a condition where the immune system of the person starts working against the person's body causing harm to the health of that person.

Complete answer:
Normally Immune system is responsible for the maintenance and improvement of health by producing antibodies against any particles or infection that enters the body which does not naturally belong to the body.
This feature of the immune system fails in case of autoimmune disease where the immune system works abnormally and it can no longer distinguish the self and non-self cells that means it can no longer identify which cell originally belong to the body and which cell is just a foreign particle or an infection which leads to the production of antibodies against both useful and harmful cells.

The actual reason for auto-immune diseases is not known but research and experiments say that mostly this disease is provoked by infectious agents and sometimes environmental factors are also responsible for this disorder.
There are many examples of autoimmune diseases that are Multiple sclerosis, Rheumatoid arthritis, diabetes mellitus, inflammatory bowel disease, etc.
In Rheumatoid Arthritis, the capsule of the joint becomes thick due to which the joints become stiff resulting in joint pains and it also affects the RBC count of the body.

Note: There are more than 80 types of auto-immune disease and all of them have almost similar symptoms which makes it difficult to identify the auto-immune disease by only considering the symptoms. The common symptoms that are observed are Fatigue, fever, joint pain, Rashes, etc. A person can also show multiple autoimmune diseases simultaneously and body parts like joints and muscles, glands such as the pancreas and thyroid are badly affected by autoimmune diseases.