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The full explanation.
Autoimmune Side Effects (irAEs) From Immunotherapy
Why immunotherapy can cause inflammation
One type of immunotherapy, called immune checkpoint inhibitor therapy, works by taking the brakes off your immune system so it can attack cancer cells. The National Cancer Institute explains that this treatment "can trigger an immune response that causes inflammation to organs in your body."
That inflammation is the source of what are often called immune-related adverse events, or irAEs. It is not an allergy and it is not the cancer getting worse. It is your own immune system acting on healthy tissue as well as on the tumor. Because the immune system can reach anywhere, these side effects can show up almost anywhere.
Which organ systems can be affected
The NCI groups them by how often they happen.
More common. Inflammation involving the digestive system, the endocrine system (the glands that make hormones), the joints and muscles, the respiratory system, and the skin.
Less common. Inflammation involving the blood, eyes, heart, kidneys, liver, and nervous system.
What the symptoms look like
Digestive system. Stomach or abdominal pain, diarrhea, and black or bloody stools. Mild inflammation of the liver (hepatitis) may show up on blood tests before you feel anything.
Endocrine system. The symptoms depend on which gland is inflamed.
- Pituitary gland: headaches and fatigue.
- Adrenal glands: fatigue, muscle weakness, loss of appetite, weight loss, and abdominal pain.
- Thyroid gland: an underactive thyroid can cause weakness, constipation, dry skin, weight gain, and feeling cold. An overactive thyroid can cause diarrhea, weight loss, sweating, and feeling hot.
- Pancreas: severe abdominal pain, nausea, and vomiting.
Joints and muscles. Joint and muscle pain, weakness, and other rheumatologic problems.
Respiratory system. Shortness of breath and severe cough.
Nervous system. Tingling or numbness in the hands, feet, or face. Inflammation of the brain can cause, in the NCI's words, "mild flu-like symptoms or more serious side effects, such as a sudden and high fever, confusion, hallucinations, seizures, and vomiting."
Notice that some of these overlap with everyday illness. Fatigue, a cough, loose stools, and aching joints all have ordinary explanations most of the time. During checkpoint inhibitor treatment, they still get reported, because the treatment changes what those symptoms might mean.
How urgently to report them
This is the part to settle before you need it. The NCI's guidance is that the answer differs by person and by symptom: "Your oncologist may advise you to call him or her first about inflammatory symptoms, or you may be advised to seek emergency medical care."
So ask your oncologist, in advance, to confirm:
- which side effects mean calling the office, and which mean going for urgent or emergency care
- where you should go for urgent medical care
- when side effects might start, and how long they may last
The NCI also advises carrying documentation of your treatment so you can share it with emergency room doctors. An ER team that does not know you are on a checkpoint inhibitor may not immediately recognize what they are looking at. Written proof of your treatment changes that conversation.
Why steroids are used
When these side effects need treatment, the NCI notes they are "sometimes managed with medicines, such as steroids, that work to slow down an overactive immune response."
That is the whole logic. The problem is an immune system that has become too active against your own tissue, so the treatment is a medicine that turns that activity down. Steroids are not being given for the cancer, and they are not a sign that treatment has failed. They are aimed at the inflammation.
Take steroids exactly as prescribed, including the schedule for coming off them, and ask your team before changing any dose.
Questions worth asking
- Which of my organs should we be watching most closely on this drug?
- Which symptoms should make me call you, and which should send me to the ER?
- What blood tests are you checking between cycles, and what would they show?
- If I need steroids, how long would I take them and what should I expect?
- What documentation should I carry about my treatment?
Source
National Cancer Institute, Immunotherapy and Organ-Related Inflammation. Accessed July 28, 2026.
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