In this article you will learn:
- What immunotherapy is and how it differs from other cancer treatments
- Which types of immunotherapy exist and how each one works
- Why cancer cells can hide from the immune system – and how immunotherapy addresses this
- What role biomarkers such as PD-L1 and MSI-H play before treatment starts
- What side effects immunotherapy can cause and when to report them
- How immunotherapy clinical trials are designed and how patients can take part
- Where to search for a recruiting trial in Europe and what you can ask your cancer team
What is immunotherapy?
Immunotherapy is a cancer treatment that uses the immune system to find and attack cancer cells. It works by helping the immune system recognise and attack cancer cells [1]. Immunotherapy does not treat the cancer directly – it aims to enhance the body's immune response and stop the cancer from evading the immune system [2].
The idea is over 130 years old. The first recorded case of immunotherapy being used to treat cancer was by Dr William B Coley in 1891 . He deliberately infected a patient with terminal bone cancer with carefully chosen bacteria, and the patient survived another eight years [3].
Immunotherapy is not a single treatment. It is a group of treatments that work in different ways, and your cancer team may call a drug by its name, its mechanism or both. For example, nivolumab may be described as immunotherapy, targeted therapy, a checkpoint inhibitor or a monoclonal antibody [4].
Why does the immune system not always destroy cancer on its own?
The immune system distinguishes the body's own normal cells ('self') from cells and substances that are foreign ('non-self'). Every cell in the body carries distinctive surface proteins that tell the immune system they belong there . Proteins recognised by the immune system are called antigens [2].
Bacteria and viruses look very different from healthy cells, so the immune system recognises them easily. Tumour cells started as healthy cells and can look a lot like them. As a result, the body may have a harder time recognising tumour cells as foreign . In other cases, the immune system recognises a tumour antigen but cannot mount a response strong enough to destroy the tumour [2].
Cancer cells can also make it hard for the immune system to find them. Some cancer cells manage to hide or trick the immune system, and the immune system eventually cannot handle the problem alone [3].
What are the main types of immunotherapy?
Immunotherapy includes several distinct families of treatments. The main types you may see named in a treatment plan or a trial are listed below [1].
- Checkpoint inhibitors – a type of immunotherapy that blocks different checkpoint proteins. Examples include pembrolizumab (Keytruda) and ipilimumab (Yervoy) [1].
- Monoclonal antibodies (mAbs) – proteins designed to attach to one specific receptor. They may be called a targeted cancer drug or immunotherapy, because they can act in both ways [1].
- CAR T-cell therapy (also called adoptive cell transfer) – a possible treatment for some children and adults with leukaemia, and for some adults with lymphoma [1].
- Vaccines to treat cancer – vaccines that train the immune system to find and attack cancer cells, distinct from vaccines that protect against disease [1].
- Cytokines – naturally occurring proteins used as immunotherapy to treat a group of blood cancers called myeloproliferative neoplasms [1].
- Immune system modulators – immunotherapy drugs given as tablets that help the immune system attack and destroy cancer cells; examples include lenalidomide (Revlimid), pomalidomide (Imnovid) and thalidomide [4].
How do checkpoint inhibitors work?
Checkpoint inhibitors are a type of monoclonal antibody that affects lymphocytes – white blood cells that can attack another cell, such as a cancer cell . When lymphocytes are active, they can attack a cancer cell. If they receive a certain signal from the other cell, they switch off and become inactive [4].
Checkpoint inhibitors block the signals that switch off lymphocytes, attaching to either the cancer cell or the lymphocyte. This means the lymphocyte stays active and can attack the cancer cell [4].
How are checkpoint inhibitors used in the European Union?
Two checkpoint inhibitors you may hear about by name are authorised in the European Union. The first active substance is pembrolizumab, marketed as Keytruda [5]. The second is nivolumab, marketed as Opdivo [6].
Pembrolizumab (Keytruda) – how it works and how it is given
Pembrolizumab is a monoclonal antibody, a protein designed to recognise and block a receptor called PD-1 . Some cancers make a protein called PD-L1 that combines with PD-1 to switch off the activity of certain immune cells, preventing them from attacking the cancer. By blocking PD-1, pembrolizumab stops the cancer switching off these immune cells, increasing the immune system's ability to kill the cancer cells [5].
Keytruda can only be obtained with a prescription, and treatment must be started and supervised by a doctor experienced in treating cancer. It is given as an infusion into a vein over 30 minutes every three or six weeks. It is also available as an injection under the skin in the abdomen or thigh, given as a 1-minute injection every 3 weeks or a 2-minute injection every 6 weeks. Patients can switch between formulations at their next scheduled dose . The doctor may delay doses if certain side effects occur, or stop treatment altogether if side effects are severe [5].
Keytruda is authorised to treat a long list of cancers, including melanoma, non-small cell lung cancer (NSCLC), classical Hodgkin lymphoma, urothelial cancer, head and neck squamous cell carcinoma (HNSCC), renal cell carcinoma, oesophageal cancer, gastric and gastro-oesophageal junction adenocarcinoma, triple-negative breast cancer, endometrial carcinoma, cervical cancer, and epithelial ovarian, fallopian tube and primary peritoneal cancer . It is also used in children aged 3 years and older with classical Hodgkin lymphoma, and in adolescents aged 12 years and older with melanoma [5].
Nivolumab (Opdivo) – how it works and how it is given
Nivolumab is a monoclonal antibody designed to attach to a receptor called PD-1 found on cells of the immune system called T cells. Cancer cells can make proteins (PD-L1 and PD-L2) that attach to this receptor and switch off the activity of the T cells, preventing them from attacking the cancer. By attaching to the receptor, nivolumab prevents PD-L1 and PD-L2 from switching off the T cells, increasing the immune system's ability to kill cancer cells [6].
Opdivo is given as an infusion into a vein or as an injection under the skin. The dose and how often it is given depends on the condition it is being used for and whether it is used alone or with other cancer medicines. As with pembrolizumab, the doctor may delay doses or stop treatment altogether for certain severe side effects [6].
Opdivo is authorised for melanoma, NSCLC, advanced renal cell carcinoma, classical Hodgkin lymphoma, squamous cell cancer of the head and neck, urothelial cancer, malignant pleural mesothelioma, MSI-H or dMMR colorectal cancer, squamous oesophageal cancer, oesophageal and gastro-oesophageal junction cancer after chemotherapy, radiotherapy and surgery, gastric, gastro-oesophageal junction or oesophageal adenocarcinoma, and hepatocellular carcinoma in previously untreated patients . It is used in adolescents from 12 years of age for melanoma and advanced classical Hodgkin lymphoma, and in patients aged 5 to 30 years with classical Hodgkin lymphoma whose cancer did not respond to, or came back after, one previous treatment [6].
What tests are needed before starting some immunotherapies?
For some cancers, checkpoint inhibitors are only given to patients whose tumours produce certain levels of a protein known as PD-L1, or are determined to be microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) [5]. The same applies to nivolumab [6]. In some cases, tests to check PD-L1 levels or MSI-H/dMMR tumour status are needed before starting treatment with pembrolizumab [5].
Pembrolizumab can be used on its own or in combination with other cancer medicines, such as lenvatinib, axitinib or enfortumab vedotin, chemotherapy alone, or chemotherapy combined with trastuzumab or bevacizumab, or chemotherapy combined with radiotherapy . In some patients with triple-negative breast cancer, NSCLC, head and neck cancer and urothelial cancer, it can be given before surgery (neoadjuvant treatment) and after surgery (adjuvant treatment) [5].
What side effects can immunotherapy cause?
Many immunotherapy side effects happen when the immune system that is revved up to act against the cancer also acts against healthy cells and tissues in the body . Different people have different side effects. Which ones you have, and how they make you feel, depends on how healthy you are before treatment, your type of cancer, how advanced your cancer is, and the type and dose of immunotherapy you are getting [7].
Side effects can occur at any point during and after treatment. Doctors and nurses cannot know for certain when or if side effects will occur, or how serious they will be, so it is important to talk with your doctors and nurses about what signs to look for and what to do if problems start [7].
Side effects common to all types of immunotherapy can include skin reactions at the needle site such as pain, swelling, soreness, redness, itchiness and rash . You may also have flu-like symptoms: fever, chills, weakness, dizziness, nausea or vomiting, muscle or joint aches, fatigue, headache, trouble breathing, and low or high blood pressure [7].
Other side effects might include swelling and weight gain from retaining fluid, heart palpitations, sinus congestion, diarrhoea, infection and organ inflammation . Some types of immunotherapy may cause severe or fatal allergic and inflammation-related reactions, but these reactions are rare . Certain side effects might happen depending on the type of immunotherapy you receive [7].
How are immunotherapy clinical trials run?
Immunotherapy is an active area of research, and newer types of immune treatments are now being studied [8]. Cancer vaccines are usually only available as part of a clinical trial (research trial) [4].
What do immunotherapy trials measure?
Trials of checkpoint inhibitors measure how long patients live and how their tumours respond. In one study of Opdivo in 418 previously untreated patients with advanced melanoma, 73% of patients treated with Opdivo were alive after 12 months, compared with 42% of patients given dacarbazine . In a second study of 405 patients with advanced melanoma whose disease had got worse despite previous treatment, around 32% (38 out of 120) of patients given Opdivo responded to treatment and had a reduction in their tumours, compared with about 11% (5 out of 47) in the comparison group [6].
How can you search for a recruiting trial?
Trials recruiting in Europe can also be searched on clinicaltrials.eu. When you look for a study, check the registry record rather than a news story, and ask your cancer team whether the trial is open to people with your type and stage of cancer.
What should you ask your cancer team before joining a trial?
Because different people have different side effects, and side effects can occur at any point during and after treatment, ask your doctors and nurses what signs to look for and what to do if you start to have problems [7]. Also ask whether the treatment being offered is already authorised in the European Union or is available only within a trial, and whether any test (such as PD-L1 or MSI-H/dMMR status) is required before starting [5].
When should you contact your doctor or nurse?
Contact your cancer team if you develop any of the side effects listed above, at any point during or after immunotherapy treatment. Side effects can occur at any point during and after treatment, and doctors and nurses cannot know in advance when or if they will occur or how serious they will be . This includes skin reactions, flu-like symptoms, diarrhoea, infection, organ inflammation, heart palpitations and fluid retention [7].
Summary
Immunotherapy uses the immune system to find and attack cancer cells, and it includes checkpoint inhibitors, monoclonal antibodies, CAR T-cell therapy, cancer vaccines, cytokines and immune system modulators [1]. Two checkpoint inhibitors – pembrolizumab and nivolumab – are authorised in the European Union and given by infusion or injection under the skin under the supervision of a doctor experienced in cancer treatment [5] [6].
Three practical steps: ask your team which biomarker test (PD-L1, MSI-H or dMMR) applies to your cancer before treatment starts [5]; ask what side effects to watch for and whom to call, because effects can appear at any time during and after treatment [7]; and if you are considering a trial, check the registry record – trials recruiting in Europe can also be searched on clinicaltrials.eu.
❓ What is immunotherapy in simple terms?
Immunotherapy is a cancer treatment that uses your immune system to fight cancer. It works by helping the immune system recognise and attack cancer cells 1. It does not attack the cancer directly – it aims to enhance the body's immune response and stop the cancer from evading the immune system 2.
❓ How do checkpoint inhibitors work?
Checkpoint inhibitors are a type of monoclonal antibody that affects lymphocytes – white blood cells that can attack cancer cells. When lymphocytes receive a certain signal from another cell, they switch off and stop attacking. Checkpoint inhibitors block those signals by attaching either to the cancer cell or to the lymphocyte, so the lymphocyte stays active and can attack the cancer cell 4.
❓ How is pembrolizumab (Keytruda) given?
Keytruda is given as an infusion into a vein over 30 minutes every three or six weeks, or as an injection under the skin in the abdomen or thigh – a 1-minute injection every 3 weeks, or a 2-minute injection every 6 weeks. Patients can switch between formulations at their next scheduled dose. Treatment must be started and supervised by a doctor experienced in treating cancer 5.
❓ What common side effects does immunotherapy cause?
Common side effects across immunotherapy types include skin reactions at the needle site – pain, swelling, soreness, redness, itchiness and rash – and flu-like symptoms such as fever, chills, weakness, dizziness, nausea or vomiting, muscle or joint aches, fatigue, headache, trouble breathing, and low or high blood pressure. Other effects can include fluid retention, heart palpitations, sinus congestion, diarrhoea, infection and organ inflammation 7.
❓ Why do some patients need a PD-L1 or MSI-H test before treatment?
For some cancers, checkpoint inhibitors are only given to patients whose tumours produce certain levels of a protein known as PD-L1, or whose cancer is described as microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR). In some cases, tests to check PD-L1 levels or MSI-H/dMMR tumour status are needed before starting treatment 5.
❓ Can cancer vaccines be prescribed outside a clinical trial?
Vaccines to treat cancer train the immune system to find and attack cancer cells, and scientists and doctors are researching new vaccines that target cancer cells. They may be used to treat several types of cancer, but they are usually only available as part of a clinical trial (research trial) 4.
❓ How well did nivolumab work in the melanoma studies?
In one study of 418 previously untreated patients with advanced melanoma, 73% of patients treated with Opdivo were alive after 12 months, compared with 42% of patients given dacarbazine. In a second study, 405 patients whose disease had got worse despite previous treatment were followed up for at least 6 months – around 32% (38 out of 120) of patients given Opdivo responded to treatment, compared with about 11% (5 out of 47) in the comparison group 6.
- [1] Targeted cancer drugs and immunotherapy (accessed 02 June 2026) — https://www.cancerresearchuk.org/about-cancer/treatment/immunotherapy
- [2] Immunotherapy | LUNGevity Foundation (accessed 02 June 2026) — https://www.lungevity.org/patients-care-partners/navigating-your-diagnosis/treatment-options/immunotherapy
- [3] What are the seven main types of cancer immunotherapy? (accessed 02 June 2026) — https://www.cclg.org.uk/news-updates/what-are-seven-main-types-cancer-immunotherapy
- [4] Types of immunotherapy for cancer (accessed 02 June 2026) — https://www.macmillan.org.uk/cancer-information-and-support/treatment/types-of-treatment/immunotherapy/types
- [5] Keytruda | European Medicines Agency (EMA) (accessed 02 June 2026) — https://www.ema.europa.eu/en/medicines/human/EPAR/keytruda
- [6] Opdivo | European Medicines Agency (EMA) (accessed 02 June 2026) — https://www.ema.europa.eu/en/medicines/human/EPAR/opdivo
- [7] Side Effects of Immunotherapy (accessed 02 June 2026) — https://www.cancer.gov/about-cancer/treatment/types/immunotherapy/side-effects
- [8] Immunotherapy: The future of cancer treatment - PMC — PubMed Central (accessed 06.10.2026) — https://pmc.ncbi.nlm.nih.gov/articles/PMC8491352/
