Chemotherapy and immunotherapy are two major forms of systemic cancer treatment, but they represent fundamentally different approaches to fighting the disease. The core distinction lies in their mechanism of action: chemotherapy uses chemical agents to directly kill cells, while immunotherapy stimulates the body’s natural defenses. While both treatments aim to control cancer growth, their methods of attack, specificity, and side effect profiles vary significantly.
How Chemotherapy Attacks Cancer
Chemotherapy functions primarily by using cytotoxic drugs—agents that are toxic to cells—to stop cancer from growing and dividing. These chemicals circulate throughout the entire body, making chemotherapy a systemic treatment useful for cancers that have spread. The drugs are designed to interfere with the cell cycle, such as by damaging the cell’s DNA or disrupting mitosis (cell division).
Cancer cells are the primary target because they divide much more rapidly than most healthy cells. However, the drugs lack the ability to distinguish between a rapidly dividing cancer cell and a rapidly dividing healthy cell. This lack of specificity is why chemotherapy affects healthy tissues like hair follicles, the lining of the digestive tract, and blood-forming cells in the bone marrow. Damage to these fast-growing healthy cells causes the commonly known side effects.
Immunotherapy: Harnessing the Body’s Defenses
Immunotherapy operates on an entirely different principle, focusing on activating the patient’s own immune system to recognize and destroy cancer cells. Rather than attacking the cancer directly, it uses biological agents to enhance the natural anti-tumor response. This approach capitalizes on the immune system’s inherent ability to distinguish between “self” and “non-self” and to launch highly specific attacks.
A prominent type of immunotherapy involves immune checkpoint inhibitors, which essentially remove the “brakes” that cancer cells place on the immune system’s T-cells. Proteins like PD-1 and CTLA-4 normally function as checkpoints to prevent the immune system from attacking healthy tissue. Cancer cells exploit these checkpoints, but the inhibitor drugs block this interaction, allowing the T-cells to become fully activated and target the tumor. This mechanism aims to create a highly targeted and durable anti-cancer response.
Comparing Treatment Approaches and Effects
The core difference is their target: chemotherapy focuses on the cancer cell itself, while immunotherapy focuses on the immune system. This distinction leads to variations in specificity and the nature of side effects. Chemotherapy is non-specific, affecting all fast-dividing cells, resulting in systemic side effects like hair loss, nausea, vomiting, and myelosuppression (suppression of bone marrow activity).
Immunotherapy is more targeted, but its side effects are related to an overactive immune response. Since the treatment stimulates T-cells, it can lead to immune-related adverse events, where the activated immune system mistakenly attacks healthy organs. These side effects often manifest as inflammation in various parts of the body, such as the lungs (pneumonitis), colon (colitis), or liver (hepatitis).
Chemotherapy’s effect is often rapid, leading to immediate tumor shrinkage, but the response may not be long-lasting if cancer cells develop resistance. Immunotherapy may take longer to show a significant effect because the immune response needs time to build up. However, once activated, the immune system can sometimes lead to longer-term control of the disease, resulting in durable remission for some patients.
When Chemotherapy and Immunotherapy Work Together
Despite their differences, chemotherapy and immunotherapy are frequently combined in chemoimmunotherapy, leveraging the strengths of both approaches. This combination is designed to work synergistically, aiming for a more comprehensive outcome than either treatment alone. The cytotoxic effect of chemotherapy can reduce the tumor’s size (debulking), making the remaining cancer cells a smaller target for the immune system.
Chemotherapy can also induce immunogenic cell death, causing cancer cells to release substances that make them more recognizable to T-cells. This process effectively turns the dying tumor cells into an in situ vaccine, priming the immune system for a more robust attack facilitated by the immunotherapy. This dual-action strategy has shown success in treating certain advanced cancers, such as non-small cell lung cancer, where the combination has significantly improved patient survival rates.