Immunotherapy is not considered chemotherapy, despite both being drug-based treatments for cancer. The distinction lies in their fundamental approach to fighting the disease. Chemotherapy uses potent chemical agents to directly attack cancerous cells. Immunotherapy is a newer class of treatment that relies on the body’s own defense mechanisms. Both methods aim to control or eliminate cancer, but they are categorized as fundamentally different therapeutic classes in modern oncology.
Mechanism of Action: Cytotoxic vs. Immunomodulatory
Chemotherapy, often referred to as “chemo,” functions through a cytotoxic mechanism, meaning it is directly toxic to cells. The drugs are designed to disrupt the rapid cell division that characterizes most cancer cells. They interfere with processes like DNA replication or cell structure formation during mitosis, which ultimately leads to cell death. Because these agents target all rapidly dividing cells, they are non-specific and affect healthy tissues, such as cells in hair follicles, the lining of the digestive tract, and the bone marrow. This indiscriminate action is the source of many common chemotherapy side effects.
In contrast, immunotherapy employs an immunomodulatory mechanism; it does not directly kill cancer cells. This treatment works by activating or enhancing the patient’s own immune system to specifically recognize and attack the tumor. A major type, called checkpoint inhibitors, blocks proteins that cancer uses to “turn off” the immune response, effectively releasing the brakes on the body’s defenses. Other forms, such as CAR T-cell therapy, involve genetically modifying a patient’s T-cells to specifically target and destroy cancer cells. The goal is to create a targeted and sustained immune response without the generalized damage of cytotoxic drugs.
Key Differences in Patient Experience and Side Effects
The difference in mechanism leads to a distinct patient experience and side effect profile between the two treatments. Traditional chemotherapy’s non-specific attack results in systemic and predictable side effects. These commonly include hair loss, nausea, vomiting, and myelosuppression (suppression of bone marrow activity leading to low blood cell counts). These effects are often immediate and cyclical, peaking shortly after drug administration.
Immunotherapy side effects are related to an overactive immune system and are known as immune-related adverse events (irAEs). Since the treatment stimulates the body’s defenses, it can lead to inflammation that mimics autoimmune conditions. These inflammatory responses can affect nearly any organ, causing conditions like colitis, pneumonitis, and hepatitis.
The nature of these side effects is fundamentally different from those caused by generalized cell death. Managing irAEs often involves using corticosteroids to suppress the overly aggressive immune response. This is a different approach than the supportive care used for chemotherapy-related side effects. Administration cycles can also vary, with some immunotherapies given less frequently than typical chemotherapy regimens.
Why Combination Therapy is Increasingly Common
Despite their distinct mechanisms, chemotherapy and immunotherapy are frequently used together in a strategy known as chemoimmunotherapy. This approach is gaining traction in modern oncology because it aims to exploit the strengths of both treatments simultaneously. The combination is designed to create a synergistic effect, where the combined impact is greater than the sum of the individual treatments.
Chemotherapy contributes to this synergy by inducing immunogenic cell death. This process causes dying cancer cells to release signals that make them more visible to the immune system. This acts as an internal alarm, providing the immune system with more targets, which is then amplified by the immunotherapy. The immunotherapy, such as a checkpoint inhibitor, ensures the newly activated immune cells can effectively engage and destroy the cancer cells without being shut down by the tumor’s defenses.
This dual-action strategy attacks the cancer from two different angles. Combining the rapid, direct cell-killing action of chemotherapy with the targeted, long-term immune activation of immunotherapy can improve overall response rates and survival for patients. Oncologists often achieve a more robust and sustained anti-tumor response.