When people hear "cancer treatment", chemotherapy is often the first thing that comes to mind. Immunotherapy is a newer and fundamentally different category - one that works through an entirely distinct mechanism. For eligible patients, it can produce outcomes that chemotherapy cannot. Understanding the difference matters when you are making decisions about your care.
How chemotherapy works
Chemotherapy drugs work by targeting rapidly dividing cells. Because cancer cells divide more quickly than most normal cells, they are disproportionately affected. The drugs interfere with cell division - either by damaging DNA directly or by disrupting the machinery cells use to replicate. Cancer cells die as a result.
The problem is that chemotherapy is not cancer-specific. It also damages fast-dividing healthy cells: those lining the gut, cells in hair follicles, and stem cells in bone marrow. This explains the characteristic side effects of chemotherapy - nausea, hair loss, fatigue, mouth sores, and vulnerability to infection. The side effect burden is often significant, though it varies considerably between different chemotherapy agents and individual patients.
How immunotherapy works
Immunotherapy does not target cancer cells directly. Instead, it modifies the immune system's ability to recognise and destroy them. Pembrolizumab specifically blocks the PD-1 checkpoint - a protein on T cells that, when activated by PD-L1 on a tumour's surface, tells the immune system to stand down. By blocking this interaction, pembrolizumab removes a brake that cancer cells have applied to immune surveillance, allowing T cells to mount a targeted attack.
Because it works by activating existing immune machinery rather than by direct cell toxicity, its side effect profile is completely different from chemotherapy. Hair loss, nausea, and bone marrow suppression are not characteristics of pembrolizumab. Instead, its risks relate to immune overactivation - the same mechanism that makes it effective can, in some cases, cause the immune system to attack healthy tissue.
Comparing side effects
Chemotherapy side effects are largely predictable and dose-related. They are often at their worst during treatment and resolve when treatment ends. Immunotherapy side effects - called immune-related adverse events (irAEs) - are less predictable and can affect almost any organ. Commonly affected systems include the thyroid, skin, liver, and lungs.
Most irAEs are mild to moderate and manageable with monitoring and, when needed, steroids. Serious irAEs affect a minority of patients. The key is early detection, which is why regular monitoring and good communication with your clinical team are built into any responsible pembrolizumab programme.
Response durability
This is where immunotherapy can be genuinely different from chemotherapy. Chemotherapy typically needs to continue as long as it is working - when it stops, the cancer usually progresses. Pembrolizumab can produce responses that persist long after treatment ends. In the KEYNOTE melanoma trials, some patients remained in remission years after their last dose. This pattern of durable benefit - seen in a meaningful minority of patients - is one of the most clinically significant characteristics of checkpoint inhibitor therapy.
Not all patients experience this. But for those who do, it represents a qualitatively different outcome from what chemotherapy can offer.
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