We never expect our immune system to turn against us, but in the case of autoimmune diseases, that's exactly what happens.

Autoimmune diseases stem from a glitch in the immune system that causes it to attack healthy parts of the body instead of performing its usual role of guarding against harmful invaders—like viruses and bacteria—that could cause disease.

Because the immune system can target any part of the body, no two autoimmune diseases are the same—they range from skin conditions like vitiligo to brain disorders like narcolepsy.

You may be surprised to learn type 1 diabetes and multiple sclerosis are some of the 100+ known autoimmune diseases that affect approximately 50 million people in the US.

We asked Cedars-Sinai doctors to tell us about a few familiar conditions caused by the immune system.

Read: Second Opinion Identifies Rare Immune Disorder

Unlike type 2 diabetes, which has been linked to factors like body weight and activity level, type 1 diabetes is not caused by lifestyle choices.

Type 1 diabetes is an autoimmune disease that attacks healthy cells in the pancreas, affecting its ability to create insulin.

"Autoantibodies are created by the immune system when it fails to distinguish between healthy cells and perceived invaders," says Dr. Bahareh Schweiger, director of pediatric endocrinology at Cedars-Sinai. "These autoantibodies then damage the healthy cells that make insulin."

Without insulin in the blood, the body cannot process sugar for energy.

According to Dr. Schweiger, type 1 diabetes warning signs include increased thirst and urination, abdominal pain, and labored breathing.

With early detection and insulin management, type 1 diabetes patients can live long, full lives.

Rheumatoid arthritis (RA) is an autoimmune disease that attacks the joints.

"RA first presents itself as painful swelling in multiple joints, typically in the hands, wrists, and toes," says Dr. Janet White, medical director of the integrative health program at Cedars-Sinai.

"In the case of RA, the body attacks healthy cells in the joints, which results in inflammation. This inflammation, if left untreated, can lead to arthritis."

Depending on the severity of the disease, doctors may treat RA with anti-inflammatories (such as ibuprofen), steroids—which have a stronger anti-inflammatory effect—and disease-modifying antirheumatic drugs.

"Early treatment of RA can help prevent permanent destruction of the joints," says Dr. White.

Multiple sclerosis (MS) is a condition where the immune system mistakenly targets the central nervous system.

"Instead of the immune system doing its usual job, in MS it attacks the optic nerve, brain, and spinal cord—the 3 components of the central nervous system," says Dr. Marwa Kaisey, assistant professor in the Cedars-Sinai Department of Neurology.

"MS is sometimes called a 'hidden disease,' and diagnosis can be tricky," says Dr. Kaisey. "But a common symptom early on is optic neuritis, which is inflammation in the nerve connecting the eye and the brain. The resulting blurred vision lasts days to months."

Current MS disease-modifying treatments don't cure existing symptoms, but they help delay the progression of MS.

"The average person with MS, if left untreated, will reach a stage called progressive MS, which is characterized by a slow, progressive decline in walking function," says Dr. Kaisey.

"However, some patients, especially those whose MS is treated early, will never need a walking aid like a cane."

Celiac disease is an autoimmune disease that targets the small intestine and is triggered by foods containing gluten—a protein found in wheat, rye, and barley.

"Patients experience abdominal pain, bloating, and diarrhea when eating gluten-rich foods," says gastroenterologist Dr. Deena Midani.

The cornerstone of treating celiac disease is strictly adhering to a gluten-free diet.