How Does the Immune System Fight Cancer Cells? A Simple Explanation

If the immune system protects us, why doesn’t it stop cancer? It is an understandable question, especially when you or someone you love is facing a diagnosis.

The answer is that it can recognise and attack some cancer cells, but it cannot stop every cancer from growing. Cancer begins in the body’s own cells, making it harder to tell which cells pose a threat. Some cancer cells also interfere with the immune response. Developing cancer does not necessarily mean that your immune system is-3 weak.

What does the immune system normally do?

Your immune system is made up of cells, tissues and organs that work together to protect you. White blood cells, antibodies and lymph nodes all have a part to play.

These cells communicate using chemical signals. Some identify a possible threat, others help organise a response, and others keep that response under control. Recognising something unusual is only the first step. Immune cells also need the right signals before they act.

How does the immune system recognise cancer cells?

As cells become cancerous, the proteins inside them may change. They may also produce more of certain proteins than healthy cells do. These differences can give the immune system something to recognise. Such targets are called antigens.

T cells are white blood cells that help with this recognition. Small protein fragments can be displayed on a cell’s surface in structures called MHC molecules. A T cell with a matching receptor may recognise that protein fragment when it is presented by an MHC molecule.

Other immune cells, including dendritic cells, can pick up material from a tumour and show it to T cells. This helps start a response. Once activated, some T cells, called cytotoxic or killer T cells, may destroy cancer cells displaying the matching target.

The basic sequence is:

Changes in a cancer cell → cancer-related signal → immune recognition → T-cell response → cancer cell may be attacked.

The word “may” matters. Recognition does not always lead to an attack. Other signals, including those around the tumour, can affect what happens next.

If the immune system can fight cancer, why does cancer develop?

The immune system does not catch every abnormal cell. Some cancer cells also have features that help them survive an immune response.

For example, they may:

• Become harder for immune cells to recognise.

• Produce proteins that reduce immune-cell activity.

• Change nearby cells in ways that make an immune attack less effective.

These changes help explain why a tumour can grow even when other parts of the immune system are working normally.

How do cancer cells hide from the immune system?

Your immune system needs to know when to act and when to hold back. Checkpoints help it do this, protecting healthy tissue from an excessive immune response. Some cancer cells can take advantage of these controls.

One example involves two proteins, PD-1 and PD-L1. PD-1 is found on T cells. When PD-L1 attaches to it, the T cell gets a signal to reduce its activity. Cancer cells that produce large amounts of PD-L1 may therefore be better able to avoid attack.

Another protein, CTLA-4, helps hold back T-cell activity through a different pathway. Researchers have developed medicines that block certain checkpoint signals. For some patients, this gives T cells a better chance of attacking the cancer.

How does immunotherapy help the immune system fight cancer?

Immunotherapy refers to a group of cancer treatments that help the immune system recognise or act against cancer. Different treatments do this in different ways. It is more specific than simply making someone’s immunity ‘stronger’.6

Immune checkpoint inhibitors

These medicines block checkpoint proteins such as PD-1, PD-L1 or CTLA-4. In some patients, releasing these brakes helps T cells respond to cancer.

T-cell therapies

A patient’s immune cells are collected, selected or changed in a laboratory, and grown in larger numbers before being returned. With CAR T-cell therapy, the cells are genetically changed to recognise a particular target on cancer cells. Specialist assessment and close monitoring are essential.

Cancer treatment vaccines

These vaccines help the immune system respond to cancer-related targets in someone who already has cancer. They are different from vaccines that prevent infections linked to cancer. Some cancer treatment vaccines are used in specific clinical situations, while many other vaccine approaches are still being studied. 

Monoclonal antibodies

These proteins are made in a laboratory to attach to particular targets. Some help mark cancer cells for immune attack; others bring T cells closer to cancer cells. Not every monoclonal antibody treatment is immunotherapy.

Your oncology team considers which treatment, if any, fits your diagnosis and health needs.

Does immunotherapy work for everyone?

How well immunotherapy works varies from person to person. Its suitability and effectiveness may depend on the type and stage of cancer, the tumour’s characteristics, previous treatments, overall health and how the body responds to therapy.

Even two people with the same type of cancer may need different treatments because their tumours can have different features. Tests for biomarkers, which are features of the cancer that may help guide treatment, can sometimes support this decision. However, a matching biomarker result does not guarantee that immunotherapy will work. Your oncology team can interpret the results and advise whether it may be suitable for you.

Can you boost your immune system to fight cancer naturally?

Wanting to take an active role in your health is understandable. A healthy lifestyle and good eating habits can support normal immune function, strengthen the body and help manage some treatment-related side effects.

Eating well can also help you maintain your strength during treatment. The aim is to give your body the nourishment it needs alongside the treatment prescribed for your cancer.

If you take supplements or are considering complementary care, let your healthcare team know. Sharing what you use or would like to try helps them check whether it is suitable alongside your treatment.

Does having cancer mean your immune system is weak?

Not necessarily. A cancer diagnosis alone does not tell you how well your immune system is working.

Some cancers and treatments can make it harder for your body to fight infections. Your care team can explain what this means for you and suggest practical precautions based on your treatment and health needs. 

Why can’t we simply make the immune system as strong as possible?

The immune system needs control as well as the ability to respond. Too much activity can harm healthy tissues.

This helps explain some immunotherapy side effects. When a treatment releases immune brakes, the response may affect healthy organs as well as cancer cells.

If you are receiving immunotherapy, tell your cancer care team promptly about new or worsening symptoms. Some immune-related side effects need early assessment and treatment. Follow your team’s instructions about when and how to contact them.

What does this mean if you have cancer?

You do not need to remember every protein name to have a useful conversation with your doctor. Focus on what the treatment could mean for you.

You could ask:

• Is immunotherapy an option for my cancer?

• Do I need any tests to help decide?

• What would we hope to achieve with treatment?

• What benefits and limitations should I understand?

• Which symptoms should I report, and whom should I contact?

• Could anything I take, including supplements, affect my treatment?

Writing down your questions before the appointment can help you cover the concerns that matter most to you.