Cancer immunotherapy: how it works, who it helps and its side effects

How the immune system fights cancer

The immune system constantly finds and destroys abnormal cells, including those with cancerous changes. Tumours, however, develop ways to hide or to switch off the immune response. One of the most important is the use of immune checkpoints – natural brakes on T cells that normally protect the body from autoimmune reactions.

Immunotherapy aims to remove these barriers or to direct immune cells straight at the tumour. Unlike chemotherapy, which acts directly on dividing cells, immunotherapy works indirectly by activating the body's own defences, so both its benefits and its side effects develop differently. It is one part of the wider range of modern cancer treatment.

Types of immunotherapy

Immune checkpoint inhibitors

These are monoclonal antibodies that block the PD-1, PD-L1 or CTLA-4 proteins and so release the brakes on T cells. They are given by drip every two to six weeks, alone or combined with chemotherapy, targeted drugs or another checkpoint inhibitor. They are by far the most widely used form of immunotherapy.

CAR-T cell therapy

The patient's T cells are collected, genetically modified in a laboratory to recognise a specific protein on cancer cells, and infused back. CAR-T therapy is used for some leukaemias, lymphomas and multiple myeloma in specialist centres.

Bispecific antibodies

These bind a T cell and a cancer cell at the same time, bringing them together. They are used for some blood cancers and certain solid tumours.

Other approaches

These include cancer vaccines (including personalised mRNA vaccines being tested in clinical trials), oncolytic viruses and older methods such as BCG treatment into the bladder for early bladder cancer.

Which cancers are treated with immunotherapy

Checkpoint inhibitors are approved by the European Medicines Agency for many cancers, and indications continue to expand. The most important include:

In some cancers, such as prostate and pancreatic cancer, immunotherapy currently has a role only in small subgroups.

Who responds to immunotherapy

Only some patients gain lasting benefit. Biomarkers help with selection:

• PD-L1 – in some cancers, such as lung cancer, higher expression in the tumour makes a response more likely.
• MSI-H/dMMR – tumours with faulty DNA repair carry many mutations and usually respond very well.
• Tumour mutational burden (TMB) – the number of mutations in the tumour, used in certain situations.

These tests are done on tumour tissue and sometimes on blood using a liquid biopsy. No biomarker predicts response with certainty. Responses can develop slowly, and occasionally a tumour appears larger on a scan before it shrinks (pseudoprogression), so results should be interpreted by an experienced oncologist.

General health also matters. People with active autoimmune diseases, organ transplants or a need for high-dose steroids may face higher risks, and the decision is made carefully case by case. Many new combinations, vaccines and cell therapies are being tested in clinical trials, and it is worth asking whether a suitable trial is open for your type and stage of cancer.

Side effects

An activated immune system can also attack healthy tissues. These immune-related adverse events are different from chemotherapy side effects. Most are mild, but some can become serious if not treated early. They can occur at any time – even months after starting or after finishing treatment.

• Skin – rash, itching, pale patches (vitiligo).
• Bowel – diarrhoea, abdominal pain, blood in the stools (colitis).
• Hormone glands – underactive or overactive thyroid; rarely the pituitary or adrenal glands, or new diabetes.
• Liver – raised liver enzymes (hepatitis).
• Lungs – cough and breathlessness (pneumonitis).
• Less often: joint pain, or effects on the kidneys, nerves or heart (myocarditis).

Treatment usually involves pausing immunotherapy and giving corticosteroids. Combination immunotherapy carries a higher risk. Some hormone problems, such as an underactive thyroid, may need lifelong hormone replacement.

When to contact a doctor

If you are having immunotherapy, contact your oncology team straight away if you develop:

• more diarrhoea than usual, or blood in your stools;
• a new cough or breathlessness;
• severe tiredness, dizziness, a bad headache or changes in vision;
• yellowing of the skin or eyes, or dark urine;
• a widespread rash or blisters;
• extreme thirst and passing urine often;
• chest pain or palpitations.

Carry an alert card or letter stating that you are on immunotherapy and show it at every appointment, including in the emergency department, because these reactions are treated differently from an ordinary infection or stomach upset.

If you are wondering whether immunotherapy could suit you, ask your oncologist whether the relevant biomarker tests have been done. If the answer is unclear, a second opinion from another oncology team can help.

Immunotherapy abroad and in Türkiye

Some patients from Bulgaria and elsewhere in Europe seek consultation or treatment at large accredited hospital groups in Istanbul – for example for extended molecular testing or access to clinical trials. Because immunotherapy continues for months and side effects can appear at any time, it is essential to know who will supervise treatment at home, how any reactions will be managed and how continuation of treatment will be arranged. Check the oncology team's experience, ask for a written treatment plan, and keep your local oncologist fully informed. New City Clinic is a health-tourism organiser that coordinates the hospital, doctor, hotel, transfers and interpreter; the choice and supervision of treatment remain entirely with the oncologists.