Cancer immunotherapy explained: unleashing the immune system on cancer

Why the immune system misses cancer

The immune system is built to spot and destroy abnormal cells, and it does eliminate many would-be cancers. But cancers that grow have found ways to hide. One key trick is exploiting the immune system's natural "brakes" — molecules called checkpoints that normally stop immune cells attacking healthy tissue. Cancers press these brakes to switch off the T-cells that would otherwise kill them. Immunotherapy is largely about releasing those brakes, or otherwise re-arming the immune response.

Checkpoint inhibitors — the breakthrough

The most widely used immunotherapies are checkpoint inhibitors — drugs that block the "off switches" (such as PD-1, PD-L1 and CTLA-4) that cancers exploit. By releasing the brake, they let T-cells recognise and attack the tumour. This class produced striking, durable responses in cancers like advanced melanoma, where outcomes had been very poor, and is now used in lung, kidney, bladder, head-and-neck and other cancers. The discovery of checkpoint control was recognised with a Nobel Prize, reflecting how fundamentally it changed cancer treatment.

Other forms of immunotherapy

Checkpoint inhibitors are one branch. Others include CAR-T cell therapy (engineering a patient's T-cells to target their cancer, used in some blood cancers); monoclonal antibodies that flag cancer cells for immune destruction or block growth signals; cancer treatment vaccines that train the immune system against tumour proteins (an active research area, including mRNA approaches); and cytokines that boost immune activity. Each works differently, and they are increasingly combined with each other or with conventional treatments.

Distinctive side effects

Because immunotherapy takes the brakes off the immune system, its side effects are different from chemotherapy. The immune system can start attacking healthy tissues, causing inflammation almost anywhere — the skin (rashes), gut (colitis/diarrhoea), lungs, liver, and hormone glands such as the thyroid. These "immune-related adverse events" range from mild to serious and need prompt recognition, often managed by dampening the immune response with steroids. This is why patients are carefully monitored and educated to report new symptoms early.

The promise and the limits

Immunotherapy has produced long-lasting remissions in cancers that were once rapidly fatal, and its use is expanding. But it does not work for everyone or every cancer — response rates vary, and predicting who will benefit is an active research challenge (biomarkers such as PD-L1 expression help but are imperfect). It can be expensive, and its side effects, while different, are real. The realistic picture is a powerful and growing option that has genuinely changed outcomes for many, used alongside — not instead of — the rest of cancer care.