Cancer immunotherapy works by helping your immune system recognize, target, and fight cancer cells. Unlike treatments that primarily act directly on cancer cells, many immunotherapies work by changing or strengthening the way your immune system responds to the cancer.
*AI-generated image - for illustration only. Clinical accuracy is not guaranteed.
Your immune system can sometimes recognize abnormal cells and destroy them. However, cancer cells can develop ways to hide from immune cells, weaken the immune response, or activate natural “brakes” that prevent immune cells from attacking. Immunotherapy is designed to overcome some of these mechanisms.
To understand how cancer immunotherapy works, it helps to look at the interaction between cancer cells and the immune system—and the different ways treatment can change that interaction. For a broader overview of the treatment, see our Cancer Immunotherapy page.
How Does the Immune System Normally Fight Cancer?
Your immune system is a network of cells, tissues, and organs that protects your body from infections and other threats. It can also recognize some abnormal cells, including certain cancer cells.
Among the important immune cells involved in this process are T cells. Some T cells can recognize specific markers, known as antigens, on abnormal cells and help destroy them.
However, cancer is more complicated than a typical infection. Cancer cells develop from your own cells, so they may not always look sufficiently different from healthy cells for the immune system to recognize them as a threat. In addition, tumors can develop mechanisms that suppress or avoid immune responses.
How Do Cancer Cells Avoid the Immune System?
Cancer cells can interfere with the body's natural immune response in several ways. For example, they may undergo changes that make them less visible to immune cells or produce signals that reduce immune activity around the tumor.
Some cancers also take advantage of immune checkpoint pathways. These pathways normally help prevent the immune system from becoming excessively active and damaging healthy tissues. Cancer cells can sometimes use these natural “brakes” to avoid being attacked.
This ability to evade immune detection is one of the reasons that simply having an immune system does not always mean the body can eliminate cancer on its own.
What Are Immune Checkpoints?
Immune checkpoints are normal control mechanisms that help regulate immune responses. They are important because the immune system needs to be powerful enough to fight threats without continuously attacking healthy tissues.
One example involves the PD-1 and PD-L1 pathway. PD-1 is a protein found on certain immune cells, including T cells. When PD-1 interacts with PD-L1, it can send a signal that reduces T-cell activity. Some cancer cells have PD-L1 on their surface and can use this pathway to reduce the immune response against them.
How Do Checkpoint Inhibitors Help?
Immune checkpoint inhibitors are designed to block certain checkpoint signals. By preventing these signals from suppressing T cells, the treatment can allow the immune cells to remain active against cancer.
For example, some medicines block PD-1 on T cells, while others block PD-L1. Other checkpoint inhibitors target CTLA-4, another protein involved in regulating T-cell activity.
In simple terms, checkpoint inhibitors can help release some of the immune system's natural “brakes,” allowing T cells to recognize and attack cancer cells more effectively.
*AI-generated image - for illustration only. Clinical accuracy is not guaranteed.
They do not generally destroy cancer cells directly in the same way that a chemotherapy drug may act; instead, they change the immune response so that the body's immune cells can do more of the work.
How Do Other Types of Immunotherapy Work?
Not all immunotherapy works by blocking immune checkpoints. Different forms of cancer immunotherapy affect the immune system in different ways.
Monoclonal Antibodies
Monoclonal antibodies are laboratory-made proteins designed to recognize specific targets. Depending on the medicine, they may attach to proteins on cancer cells or influence immune activity.
Some monoclonal antibodies help the immune system identify cancer cells more effectively, while others block signals that cancer cells use to grow or avoid immune attack. This means that not every monoclonal antibody used in cancer treatment works in exactly the same way.
T-Cell Transfer Therapy
T-cell transfer therapy uses your own immune cells to create a stronger cancer-fighting response. In some approaches, immune cells are collected from your blood or tumor and selected or modified in a laboratory. They are then multiplied and returned to your body.
CAR T-cell therapy is one example. In this treatment, certain T cells are genetically modified so they can better recognize specific targets on cancer cells. The modified cells are expanded in the laboratory and then infused back into the patient.
*AI-generated image - for illustration only. Clinical accuracy is not guaranteed.
Another approach is tumor-infiltrating lymphocyte (TIL) therapy. It uses T cells that have already entered a tumor, selects those that appear capable of recognizing the cancer, expands them in the laboratory, and returns them to the body.
Cancer Treatment Vaccines
Cancer treatment vaccines are different from vaccines that prevent infections or certain cancers. They are designed for people who already have cancer and aim to stimulate an immune response against cancer-associated targets. These vaccines may help the immune system recognize particular antigens associated with cancer cells and respond against them.
Cytokines and Other Immune-Stimulating Treatments
Cytokines are proteins that act as signals between immune cells. Certain cytokine-based treatments can stimulate or modify immune activity to help the body respond to cancer.
Other immunotherapies can also strengthen particular parts of the immune response or use different biological mechanisms to help the immune system fight cancer.
What Happens After Immunotherapy Activates the Immune Response?
The exact response depends on the type of immunotherapy, the cancer being treated, and individual factors. In general, treatment aims to improve the immune system's ability to recognize and act against cancer.
Once activated immune cells recognize their targets, they may attack and destroy cancer cells. In some situations, the immune response can continue to have an effect after the treatment itself has been administered.
However, immunotherapy does not work for every person or every type of cancer. Cancer cells can use several different mechanisms to avoid immune attack, and tumors can differ significantly from one another.
This is why doctors consider the specific cancer, its characteristics, previous treatments, and other patient factors when deciding whether immunotherapy may be appropriate.
Why Doesn't Immunotherapy Work the Same Way for Everyone?
There is no single response to immunotherapy because both cancer and the immune system vary from person to person. Factors that can influence treatment include:
- The type and stage of cancer
- Specific characteristics or biomarkers found in the tumor
- Whether the tumor expresses certain immune-related proteins
- How effectively immune cells can recognize the cancer
- Previous cancer treatments
- The person's overall health and immune status
Some cancers are more responsive to particular forms of immunotherapy than others. In some cases, testing the tumor for specific biomarkers can help doctors determine whether a particular immunotherapy may be useful.
Can Immunotherapy Be Combined With Other Cancer Treatments?
Yes. Depending on the cancer and treatment plan, immunotherapy may be used alone or alongside other treatments such as chemotherapy, radiation therapy, targeted therapy, or other immunotherapies.
The reason for combining treatments is that different therapies can affect cancer through different mechanisms. For example, one treatment may directly damage cancer cells while another helps the immune system recognize or attack them.
*AI-generated image - for illustration only. Clinical accuracy is not guaranteed.
Whether a combination is appropriate depends on the cancer type, stage, tumor characteristics, previous treatments, and the specific medicines being considered.
Why Can Immunotherapy Cause Side Effects?
Because immunotherapy changes immune activity, its effects can sometimes involve healthy tissues as well as cancer cells. For example, immune checkpoint inhibitors can cause the immune system to become more active and, in some cases, attack normal tissues.
Side effects can therefore involve organs such as the skin, bowel, lungs, liver, thyroid, or other parts of the body. The type and severity of side effects vary considerably.
Some are mild, while others can become serious and require prompt medical attention. Your cancer care team should be informed about new or unusual symptoms during treatment so they can determine whether they may be treatment-related.
*AI-generated image - for illustration only. Clinical accuracy is not guaranteed.
Does Immunotherapy Mean the Immune System Is Completely Boosted?
Not necessarily. “Boosting the immune system” is a useful simplified description, but immunotherapy is more specific than simply making the entire immune system stronger.
Different immunotherapies act on different parts of the immune response. A checkpoint inhibitor, for example, blocks a particular inhibitory signal. CAR T-cell therapy changes specific T cells so they can recognize a particular cancer target. Other treatments stimulate different immune mechanisms.
So, rather than simply increasing immunity throughout the body, cancer immunotherapy generally aims to change or direct the immune response in a way that helps fight cancer.
The Bottom Line
Cancer immunotherapy works by changing the interaction between the immune system and cancer. Some treatments remove immune “brakes,” while others help immune cells recognize cancer more effectively, modify T cells to target cancer, or stimulate specific immune responses.
The important point is that immunotherapy is not one single treatment. It is a group of therapies that use different biological mechanisms to help the immune system fight cancer. Whether a particular immunotherapy is suitable depends on the type and characteristics of the cancer, the patient's overall situation, and the treatment options available.
