How the Immune System Recognizes and Destroys Cancer Cells

Cancer Is Not Invisible to the Immune System

Cancer cells are not foreign invaders from outside the body — they are the body's own cells that have accumulated genetic mutations causing them to divide uncontrollably. Yet despite this intimacy, the immune system can and does recognize and destroy enormous numbers of abnormal cells every day. The development of a clinically detectable tumor represents, in most cases, a failure or circumvention of these immune defenses — not their complete absence.

Understanding the immune system's relationship with cancer has driven one of the most significant advances in oncology in decades: cancer immunotherapy. By understanding how the immune system normally recognizes and kills cancer cells — and how tumors evade it — researchers have developed treatments that restore and amplify that surveillance, saving lives in cancers that were previously untreatable.

The Surveillance Hypothesis: Immune Patrolling

The cancer immunosurveillance hypothesis, formulated in the 1950s and refined since, proposes that the immune system continuously monitors the body for abnormal cells and eliminates them before they can develop into tumors. Evidence for this includes the significantly higher cancer rates observed in immunocompromised individuals — those with HIV/AIDS, organ transplant recipients on immunosuppressants, and people with certain genetic immunodeficiencies.

This surveillance is imperfect. Some cancer cells evolve mechanisms to hide from or suppress immune detection. The modern understanding is captured in the three Es of cancer immunoediting: elimination (immune system destroys early cancer cells), equilibrium (a stalemate in which some cells survive), and escape (the surviving cells develop resistance and grow into a clinical tumor).

T Cells: The Primary Cancer Killers

Cytotoxic T lymphocytes (CTLs), also called CD8+ T cells, are the immune system's primary direct killers of cancer cells. They work by recognizing fragments of abnormal proteins — called neoantigens — displayed on the surface of cancer cells via molecules known as MHC class I proteins. Every nucleated cell in the body displays samples of the proteins it is producing; mutated cancer cells display abnormal fragments that mark them as different.

When a CTL's receptor binds to a neoantigen-MHC complex on a cancer cell, it initiates a killing sequence: the T cell releases perforin (a protein that punches holes in the cancer cell's membrane) and granzymes (enzymes that trigger programmed cell death, or apoptosis). A single CTL can kill multiple cancer cells sequentially before moving on.

Natural Killer Cells: A Different Strategy

Natural killer (NK) cells use a complementary detection method. Rather than looking for abnormal proteins, NK cells look for the absence of normal ones. Healthy cells display MHC class I molecules that send inhibitory signals to NK cells, essentially saying I am normal, do not kill me. Many cancer cells downregulate or lose MHC class I expression in an attempt to hide from T cells. This loss of inhibitory signal actually activates NK cells, which then kill the cancer cell.

NK cells are a critical part of the innate immune response — they do not require prior sensitization and can respond to new threats immediately. They also produce cytokines like interferon-gamma that recruit and activate other immune cells to the tumor site.

How Tumors Escape Immune Detection

Tumors are not passive targets. Through evolution and selection, cancer cells develop multiple strategies to evade immune destruction:

• Downregulation of MHC I: Reducing the display of neoantigens makes cancer cells harder for T cells to find — though this backfires by activating NK cells.
• Upregulation of checkpoint proteins: Proteins like PD-L1 on cancer cells bind to PD-1 receptors on T cells, sending an inhibitory signal that effectively tells the T cell to stop attacking. This is the normal mechanism the body uses to prevent autoimmunity, and tumors exploit it to suppress immune responses.
• Creating an immunosuppressive tumor microenvironment: Tumors recruit regulatory T cells (Tregs) and secrete cytokines that suppress the activity of other immune cells within and around the tumor.
• Antigen loss: Cancer cells that express an antigen targeted by the immune system may selectively die, leaving behind a subpopulation that does not display that antigen — an evolutionary escape.

Immunotherapy: Removing the Brakes

The insight that tumors exploit normal immune checkpoints to suppress T cell activity led to the development of checkpoint inhibitor therapies — one of the most important advances in cancer treatment of the past thirty years.

Drugs like pembrolizumab (Keytruda) and nivolumab (Opdivo) block the PD-1/PD-L1 interaction, releasing the brake on T cells that have been suppressed by the tumor. Similarly, ipilimumab blocks the CTLA-4 checkpoint, another mechanism tumors use to suppress immune activation. By releasing these brakes, checkpoint inhibitors restore the immune system's ability to attack the tumor.

These therapies have produced remarkable responses in cancers previously considered untreatable — including metastatic melanoma, certain lung cancers, and bladder cancer. However, not all patients respond, and the therapies carry a risk of immune-related adverse events: with the brakes removed, the immune system can also mistakenly attack normal tissue, causing inflammation in the lungs, gut, liver, or other organs.

CAR-T Cell Therapy

Chimeric antigen receptor T cell (CAR-T) therapy takes a more direct engineering approach. T cells are extracted from the patient's blood, genetically modified in a laboratory to express artificial receptors that recognize specific proteins on cancer cells, multiplied in large numbers, and then reinfused into the patient. These engineered T cells become precision-guided killers targeting a specific cancer antigen.

CAR-T therapy has produced complete remissions in some patients with blood cancers — including certain types of leukemia and lymphoma — who had exhausted all other treatment options. Extending this success to solid tumors remains a major challenge, as the dense, immunosuppressive environment of solid tumors limits T cell penetration and persistence.

The Future of Cancer Immunology

Research is advancing rapidly on multiple fronts: personalized cancer vaccines that target the unique neoantigens of an individual patient's tumor, combinations of checkpoint inhibitors with other therapies to overcome resistance, and better strategies for engineering T cells to penetrate solid tumors. The fundamental insight — that the immune system is the body's most powerful anti-cancer tool, and that cancer's greatest vulnerability is often its relationship with immunity — continues to guide the most promising directions in oncology.