Your body has built-in defenses that find and destroy cancer cells every day, and modern medicine has developed several powerful ways to boost or mimic that process. The short answer: immune cells (especially killer T cells and natural killer cells), chemotherapy drugs, radiation, engineered antibodies, and newer therapies like CAR-T cells and oncolytic viruses all attack cancer cells through different mechanisms. Understanding how each one works helps explain why doctors often combine them.
Killer T Cells: Your Body’s Precision Assassins
The immune system’s most direct cancer-fighting weapon is a type of white blood cell called a cytotoxic T cell (also known as a CD8 T cell). These cells patrol the body looking for anything abnormal on cell surfaces. When a T cell recognizes a cancer cell, it locks on and releases two types of toxic proteins stored in specialized granules.
The first protein, perforin, punches holes directly through the cancer cell’s outer membrane. The second group, called granzymes, are enzymes that slip through those holes and enter the cell. Once inside, granzymes trigger a chain reaction that activates the cell’s self-destruct program, a process called apoptosis. Granzyme B specifically activates an enzyme cascade that ends with the cancer cell’s own DNA being chopped apart from the inside. Both perforin and granzymes are required for effective killing. One creates the entry point, the other delivers the death signal.
Natural Killer Cells and “Missing Self” Detection
Natural killer (NK) cells take a different approach. Instead of recognizing cancer directly, they look for what’s missing. Healthy cells display a set of identity tags on their surface (called MHC class I molecules) that essentially say “I’m normal, don’t attack me.” Cancer cells and virus-infected cells often lose or reduce these tags as they mutate. NK cells have inhibitory receptors that normally detect these tags and stand down. When the tags are absent, NK cells interpret the silence as a threat and attack.
NK cells also carry activating receptors, like NKG2D, that recognize stress signals cancer cells sometimes display. So they’re reading two channels at once: the absence of “I’m healthy” signals and the presence of “I’m in trouble” signals. When the math tips toward danger, the NK cell kills.
Macrophages: The Immune System’s Cleanup Crew
Macrophages are large immune cells that engulf and digest abnormal cells, a process called phagocytosis. Cancer cells have evolved a clever defense against this. Many tumors overexpress a surface protein called CD47, which binds to a receptor on macrophages and transmits a “don’t eat me” signal. This effectively cloaks the cancer cell from being consumed.
Researchers have developed therapies that block this CD47 signal, stripping away the cancer cell’s disguise and allowing macrophages to attack. Early clinical results targeting this pathway have shown promising anticancer effects across several tumor types.
How Chemotherapy Damages Cancer Cells
Chemotherapy drugs attack cancer by exploiting the fact that cancer cells divide rapidly and often have broken repair mechanisms. Different drug classes hit cells at different stages of division. Alkylating agents, one of the oldest classes, can damage cells at any stage of growth, including cells that are temporarily resting. Antimetabolites work during the DNA-copying phase specifically, disrupting the cell’s ability to replicate its genetic material. Plant-derived drugs target the final stage of cell division, when the cell physically splits in two.
The reason chemotherapy causes side effects like hair loss and nausea is that it can’t perfectly distinguish cancer cells from healthy cells that also divide quickly, like those in hair follicles, the gut lining, and bone marrow. However, healthy cells generally have intact repair machinery, so they recover in ways cancer cells cannot. Some newer strategies exploit this difference by using protective drugs that temporarily pause healthy cell division during treatment, shielding normal tissue while cancer cells remain vulnerable.
Radiation Therapy and DNA Destruction
Radiation kills cancer cells by breaking their DNA. Ionizing radiation damages DNA in two ways: it deposits energy directly into the DNA strand, and it splits water molecules inside cells to create highly reactive chemicals called hydroxyl radicals. These radicals attack the DNA and cause double-strand breaks, the most lethal form of DNA damage a cell can sustain.
Healthy cells have functioning repair systems that can fix many of these breaks. Cancer cells, with their already unstable genetic machinery, are far less capable of repairing the damage and die as a result. Modern radiation techniques like proton therapy and intensity-modulated beams focus energy precisely on the tumor, further reducing harm to surrounding tissue.
Checkpoint Inhibitors: Releasing the Brakes
One of the biggest breakthroughs in cancer treatment has been checkpoint inhibitor therapy. Cancer cells often produce large amounts of a protein called PD-L1 on their surface. When PD-L1 binds to PD-1, a protein on T cells, it sends an “off” signal that shuts down the T cell’s attack. Essentially, the cancer cell is flashing a fake badge that tells the immune system to stand down.
Checkpoint inhibitors are drugs that block this handshake. By preventing PD-1 and PD-L1 from connecting, the “off” signal is never sent, and T cells remain active and able to kill. Another class of checkpoint inhibitor targets a different protein called CTLA-4, which also dampens T cell activity. These drugs have become standard-of-care across many cancer types and are frequently combined with each other or with chemotherapy.
Monoclonal Antibodies: Guided Missiles
Monoclonal antibodies are lab-engineered proteins designed to latch onto specific targets on cancer cells. They attack tumors in three distinct ways.
- Blocking growth signals. Some antibodies bind to receptors that cancer cells use to receive growth instructions, effectively cutting off the signal that tells the tumor to keep expanding.
- Flagging cells for immune attack. Antibodies can act as bridges, binding to the cancer cell on one end and recruiting immune cells on the other. This directs the immune system’s firepower to the exact location of the tumor.
- Delivering toxic payloads. Antibody-drug conjugates attach a chemotherapy drug or radioactive molecule to the antibody. The antibody delivers its cargo directly to the cancer cell, concentrating the toxic effect while sparing healthy tissue.
CAR-T Cell Therapy: Reprogrammed Immune Cells
CAR-T cell therapy takes a patient’s own T cells and genetically engineers them to be better cancer hunters. The process starts with a blood draw. T cells are separated out and sent to a lab, where they’re modified to produce special receptors on their surface called chimeric antigen receptors (CARs). Each CAR is built from fragments of lab-made antibodies on the outside and signaling components on the inside.
The external portion lets the T cell recognize and lock onto a specific protein found on cancer cells. The internal portion sends activation signals that tell the T cell to multiply and attack. Once infused back into the patient, these modified cells seek out cancer cells carrying the target protein and destroy them. The first CAR-T therapy approved was for children with a type of leukemia that had relapsed, and clinical trials found it eliminated leukemia in most of those patients. Several CAR-T therapies are now approved for blood cancers, with research expanding into solid tumors.
Oncolytic Viruses: Cancer-Killing Infections
Oncolytic viruses are engineered or naturally selected viruses that infect and replicate inside cancer cells but not healthy ones. The virus enters the cancer cell, hijacks its machinery to make copies of itself, and eventually causes the cell to burst open. This releases new virus particles that go on to infect neighboring cancer cells, along with tumor fragments that alert the immune system to the cancer’s presence.
The immune-stimulating effect is particularly valuable. When cancer cells die this way, they release proteins and signals that can prime an immune response against tumors elsewhere in the body, not just at the injection site. Some oncolytic viruses are further “armed” with therapeutic genes that amplify this systemic immune response. The viruses are modified so they can only complete their life cycle in cancer cells, which have typically lost the antiviral defenses that protect normal tissue.
Why Combination Matters
Cancer cells are genetically unstable, which means they mutate constantly and can develop resistance to any single attack. A tumor that survives chemotherapy might still be vulnerable to T cells. A cancer that hides from the immune system by displaying PD-L1 can be unmasked with a checkpoint inhibitor and then killed by the newly activated T cells. Radiation can break open tumor cells and release their contents, making them more visible to immune cells primed by immunotherapy. This is why oncologists increasingly use combinations of these approaches, layering natural immune defenses, drugs, and engineered therapies to hit cancer from multiple angles at once.