The immune system helps your body fight infections and other diseases, including cancer, by detecting and destroying abnormal cells that have become cancerous. Sometimes, the cancer cells can hide from the immune system so it doesn’t know where to find them, or your natural immune system might not be strong enough to fight the abnormal cell growth.

Immunotherapy in its various forms can either boost your immune system to help it fight the cancer, or make it easier for your immune system to identify cancer cells and destroy them, or slow their growth.

Immunotherapy can be given in different ways via injection, infusion through the veins or delivered directly into the bladder. Like other forms of treatment, immunotherapy is given over a period of time, often in cycles depending on each individual case.

There are many different kinds of immunotherapy which work in different ways.

• Cancer vaccines – medicines that trigger the body’s immune system to prevent cancer cells from developing or fight existing cancer cells.
• Monoclonal antibodies – also known as therapeutic antibodies, these are immune system proteins designed to attach to specific targets found on cancer cells so that they will be better seen and destroyed by the immune system.
• Checkpoint inhibitors – medicines that help the immune system respond more strongly to a tumour by releasing “brakes” that keep T cells (a type of white blood cell and part of the immune system) from killing cancer cells.
• CAR T-cell therapy – an emerging type of treatment in which a patient’s T-cells (a type of immune system cell) are made in the laboratory to have specific receptors that bind to their counterpart proteins on cancer cells. This allows the immune cells to attack the cancer cells when they are introduced back into the body.

Cancer cells learn to survive and grow in a hostile environment as they can hide from the immune system and avoid detection by the body’s own self defences. Once the immune system is alerted and activated, other healthy cells may be recognised as different and vulnerable to attack. This is what causes some of the side effects of immunotherapy.

How you experience immunotherapy side effects depends on how healthy you are before treatment, your type of cancer, how advanced it is, the type of therapy you are getting, and the dose.

The most common side effects are skin reactions at the needle site, which can include pain, swelling, soreness, redness, itchiness and a rash.

You may have flu-like symptoms, such as:

• Fever
• Chills
• Weakness
• Dizziness
• Nausea or vomiting
• Muscle or joint aches
• Fatigue
• Headache
• Trouble breathing
• Low or high blood pressure.

Other side effects might include:

• Swelling and weight gain from fluid retention
• Heart palpitations
• Sinus congestion
• Diarrhoea
• Risk of infection.

Immunotherapies may, on very rare occasions, also cause severe or even fatal allergic reactions.

For more information about immunotherapy and its side effects, you should talk to your doctor.