What is the Difference Between Chemotherapy and Immunotherapy?

Both are used to treat cancer. Both can be effective. But they work in very different ways, and they are used for different reasons depending on the situation.

Understanding that difference is less about the drugs themselves and more about how treatment decisions are made over time.

How chemotherapy works

Chemotherapy works by directly targeting cancer cells.

The drugs are designed to damage or destroy cells that divide quickly. Cancer cells tend to divide more rapidly than normal cells, which is why chemotherapy can be effective.

The difficulty is that chemotherapy not only affects cancer. It can also affect other fast-growing cells in the body, including cells in the hair follicles, skin, blood, and digestive system. That is why side effects such as fatigue, nausea, hair loss, and a higher risk of infection can occur.

In practice, chemotherapy is often used when there is a need to reduce tumour size more quickly, control spread, or treat cancer throughout the body rather than in one specific area.

How immunotherapy works

Immunotherapy takes a different approach.

Instead of attacking cancer directly in the same way, it works by helping the immune system recognise and respond to cancer cells. Some cancers are able to avoid detection by the immune system. Immunotherapy can help remove that barrier, allowing the body to identify cancer cells as something that should be attacked.

Because of this, immunotherapy is often described as more targeted in how it works. It does not rely on damaging all rapidly dividing cells in the same way chemotherapy does.

That said, it does not work for everyone, and the response is not always immediate. In some cases, the effect builds over time rather than appearing straight away on scans.

The key difference between the two

The simplest way to think about it is this.

Chemotherapy acts directly on cancer cells. Immunotherapy works through the immune system.

That difference shapes how each treatment behaves.

Chemotherapy can lead to a more immediate reduction in tumour size. Immunotherapy may take longer to show an effect, but in some cases, the response can be more sustained.

It also changes how results are interpreted. With immunotherapy, early scans do not always follow the expected pattern, which can make things less straightforward at the start.

How these treatments are used in practice

In reality, it is rarely a case of choosing one over the other in isolation.

Some patients start with chemotherapy, particularly where there is a need for a faster response. Others may begin with immunotherapy if the cancer is likely to respond to it. In many cases, both are used together or in sequence.

This is particularly relevant in cancers such as lung cancer and melanoma, where treatment decisions are guided by tumour biology, stage, and how the disease is behaving over time.

This is where a clear approach to chemotherapy and immunotherapy becomes important. The focus is not just on selecting a drug, but on deciding when to use each option, how they fit together, and how the plan may need to change.

Side effects and what to expect

The difference in how these treatments work also affects side effects.

Chemotherapy side effects are usually related to its effect on fast-growing cells. These can include fatigue, nausea, hair loss, and an increased risk of infection.

Immunotherapy side effects are different. They are often linked to the immune system becoming more active than intended. In some cases, this can lead to inflammation affecting areas such as the skin, lungs, bowel, liver, or hormone glands.

Most side effects are manageable, but they need to be recognised early. Monitoring is part of treatment, not just to assess response, but to understand how the body is reacting.

Why the distinction matters

The difference between chemotherapy and immunotherapy is not just technical. It shapes how treatment is used and what to expect from it.

In some cancers, such as certain types of lung cancer, chemotherapy may be used first to bring the disease under control more quickly. In other cases, immunotherapy may be introduced early if the cancer is more likely to respond to it based on its biological features. There are also situations where both are used together.

So it is less about choosing one over the other and more about how they are sequenced and adjusted. A treatment that works well at the start may be adapted later, depending on how the cancer responds.

What matters is not which option sounds more advanced, but which approach fits the situation at hand.

The role of decision-making in treatment

Treatment does not usually follow a fixed path.

You might start with one approach and stay on it. Or things might shift depending on how the cancer responds, how you tolerate treatment, or what changes show up on scans.

Sometimes a treatment works well for a period, then becomes less effective. Other times, the response is slower than expected and needs time to become clear.

Decisions tend to be made step by step rather than all at once. Not rushed, but not left either.

This is where having the right consultant oncologist involved, as part of a wider clinical team, becomes important. Not just to choose a treatment, but to make sense of what is happening as things evolve.

In practice, this often comes down to having a clear, private cancer treatment plan that can adapt over time. One that reflects your situation, rather than a fixed pathway.

About Dr James Wilson

Dr James Wilson is a consultant clinical oncologist based in full-time private practice in Central London. He specialises in chemotherapy, immunotherapy, and advanced radiotherapy, with a focus on clear decision-making and continuity of care. His approach centres on developing a treatment strategy that reflects the individual patient and how their cancer behaves over time.