Very common and generally poorly understood with limited option for treatment (usually symptomatic treatment), it seems that autoimmune disease is a big challenge, for example we are talking about 24 million people (around 7%) have autoimmune diseases in USA only.
Take a look around you, you might notice like I did that you have or know someone who have autoimmune condition, I had alopecia areata once which is autoimmune mediated, my sisters daughter have Diabetes militias type 1, my grandmother have hypothyroidism most likely due to hashimoto thyroiditis (autoimmune) and my uncle have rheumatoid arthritis, my best friend have Lupus.
Unlike infectious disease which is preventable autoimmune disease is a nightmare for public health institutions, the best they can do is encouraging health care provider to have a low suspicion dealing with these diseases for early detection and management to keep it under control. 
How could body cells attacks itself?
German immunologist Paul Ehrlich is the first one described this concept as a "horror autotoxicus" In 1904, it was revisionary at that time.
It all start in your bone marrow with B cell and T cell (immune cell), in their maturation by very complicated process if any cell over activated or under activated it will be rejected immediately and removed from circulation because over activation will make the cell attack other body cell and if under activation it will be almost worthless against pathogen (immunodeficiency disorder). Immune cell should be in a very narrow range of activity to be graduated and officially declared as a police officer in a human body.
That being said you also need to know that there is many external antigen resemble human cell antigen, for example viral infection that have similar antigen to pancreatic beta cell can trigger an immune response to attack pancreatic cell also as a part defense mechanism and it can be a huge attack against pathogen and our own cell that have similar structure. We have strong immune system that keep us safe but if wrongly self activated it will make us sick instate of tiny virus for instance, some time our immune system act like burning a house to kill a rat.
Before we start autoimmune disease list lets conclude that immune system is a very restricted system with accurate slandered that should be applied to all killing cell (body police officer) in another words white blood cell, any disequilibrium can cause either Autoimmune disease or immunodeficiency disorder.
The 4 most common autoimmune diseases
Diabetes (type 1)
Beta cells in the pancreas secret Insulin, a vital hormone for metabolic process in human body, lack of insulin is fatal leading to imbalance in internal homeostasis, In DM type 1 there is autoimmune destruction to these cells lead to deficiency in insulin, it can be sever incompatible with life. Without insulin replacement therapy metabolic acidosis and multi organ failure can happened in days.
Famous theory claim that viral infection can trigger an autoimmune response (beta ceel destruction) in predisposing individuals. In a nutshell DM1 have genetic (run in family) and environmental (trigger) component.
NB : DM 1 is a different disease in term of pathophysiology than DM2, in the last there is resistant to insulin effect not deficiency. DM 1 is common in young people while the second in old.
DM usually presented with polyphagia (eat a lot), polydipsia (drink a lot), polyurea (urinate a lot) and other symptoms.
Systemic lupus erythematosus (SLE)
People call it Lupus, caused by an unknown environmental trigger, affect individuals with genetic susceptibility and already have a defects in their immune system. SLE affects most part of the body including :
Skin : cause many type of skin rash but ((butterfly rash)) is typical for Lupus
Blood : Anemia, low platelet, low White blood cell and coagulation disorder is common in SLE.
Heart : can cause Endocarditis (inflammation of heart inner lining)
Kidney : Lupus nephritis is a fancy name for kidney complication can caused by lupus, they are many and complicated conditions.
Lungs : common complication of SLE is Pleurisy which is inflammation of pleurae (membrane cover the lungs)
muscle &bone : joints pain is very common in SLE and that confuse many doctor and they misdiagnose patient with rheumatoid arthritis or osteoarthritis
SLE have variety of presentation, that’s way it is difficult to diagnosed clinically and there is special criteria should be applied to suspect Lupus, then lab test can confirm or exclude Lupus.
Rheumatoid arthritis
Family history is the most important risk factor, environmental trigger unknown, RA is a chronic autoimmune disease mostly affect synovial tissue (lines some joints and tendons sheath)
Joint pain with morning stiffness for more than hour is the most prominent symptoms and it has to be for more than 6 weeks to be consider as relevant to suspect RA. The main different between both is that RA cause destructive arthritis (joint destruction in x ray) and in long term joint deformity which not in SLE.
Like Lupus it can cause broad spectrum of complication in most body organ.
Celiac disease
Autoimmune disease affect small intestine in genetically predisposed individual (run in family), every time patient ingest gluten it trigger an autoimmune destruction to small bowl which presented with abdominal pain & distension, diarrhea and malabsorption.
Gluten: is a protein found in wheat and other grains like barley, ray and oat, there is processed food for celiac patient ((gluten free food)).
