The immune system is the body’s defense network, constantly working to protect against threats, including abnormal cells that can develop into cancer. Cancer is a complex disease that interacts with this protective system, often finding ways to circumvent its defenses. Understanding cancer’s influence on the immune system is fundamental to grasping disease progression and treatment challenges.
The Immune System’s Natural Defense Against Cancer
The body maintains a constant surveillance system, known as immunosurveillance, to detect and eliminate abnormal cells, including those with the potential to become cancerous. This process involves a coordinated effort from different immune cells that patrol the body. These cells identify changes in surface proteins on abnormal cells, which signal them as threats.
T lymphocytes (T cells) are white blood cells that directly recognize and destroy infected or cancerous cells. Natural Killer (NK) cells, part of the innate immune system, identify and eliminate transformed cells, especially those that have lost certain surface markers. Macrophages are large white blood cells that engulf and digest cellular debris and pathogens, and they also present abnormal cell components to other immune cells.
These immune cells work together, releasing cytotoxic proteins or signaling other immune components, to prevent tumor formation and growth. This initial response is a foundational aspect of the body’s protection against cancer.
Cancer’s Evasion Tactics
Cancer cells develop strategies to avoid immune detection and destruction. One way is by reducing antigen presentation, essentially “hiding” their abnormal proteins that would normally alert immune cells. This makes them less visible to patrolling T cells, preventing an effective immune response.
Another tactic involves immune checkpoint activation. Cancer cells can express molecules like PD-L1 (Programmed Death-Ligand 1) on their surface. When PD-L1 binds to PD-1 (Programmed Death-1) receptors on immune cells, particularly T cells, it sends an inhibitory signal, effectively “turning off” the immune cell’s ability to attack the cancer. This mechanism is a natural brake on the immune system, which cancer cells exploit.
Cancer cells can also create an immunosuppressive microenvironment around the tumor. They recruit and activate other cells, such as regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs), which actively suppress the activity of anti-cancer immune cells within the tumor’s vicinity. This localized suppression allows the tumor to grow unchecked.
Cancer cells also exhibit genetic instability, leading to rapid mutations. This allows them to quickly change or lose specific antigens that the immune system might have initially recognized. This constant evolution makes it difficult for the immune system to keep up, as cancer cells present new, unrecognized targets.
How Cancer Weakens Overall Immune Function
Beyond specific evasion tactics, cancer can lead to a broader, systemic weakening of the immune system, making the body more susceptible to other illnesses. Chronic inflammation, often induced by the tumor, can cause immune cells to become “exhausted” or dysfunctional. This prolonged activation can deplete their resources and reduce their ability to mount an effective response against cancer or other pathogens.
Cancer can also divert nutrients, leading to malnutrition and weakness (cachexia). This nutrient competition can impair the production and function of various immune cells, compromising the body’s overall defense capabilities.
Some cancers directly release substances that inhibit immune cell activity or production. These cancer-derived products can interfere with signaling pathways or suppress the proliferation of immune cells, contributing to a generalized state of immunosuppression.
If cancer metastasizes to the bone marrow, it can directly disrupt the production of new white blood cells and other immune components. The bone marrow is the primary site for generating immune cells, and its infiltration by cancer cells severely compromises the body’s ability to replenish its immune defenses. As a consequence, individuals with cancer often experience increased susceptibility to infections, as their weakened immune system struggles to fight off bacteria, viruses, and fungi.
Immune System Changes Due to Cancer Treatments
While cancer treatments eliminate cancerous cells, many can also have significant, often temporary, impacts on the immune system. Chemotherapy, for instance, targets rapidly dividing cells, which includes many healthy immune cells like neutrophils. This can lead to a temporary state of immunosuppression, increasing the risk of infections.
Radiation therapy, which uses high-energy rays to kill cancer cells, can also damage immune cells in the treated area. Depending on the extent and location of the radiation, there can be systemic effects on the immune system. Surgical procedures, while removing tumors, can also induce a temporary, stress-related suppression of immune function.
Corticosteroids, often prescribed alongside chemotherapy or to manage inflammation and other symptoms, are powerful immunosuppressants. Their use can further reduce the activity of immune cells, contributing to a weakened immune response.
Some newer cancer therapies, such as immunotherapies like immune checkpoint inhibitors, work by harnessing or boosting the immune system to fight cancer. However, the primary focus of many conventional treatments is on directly targeting cancer cells, which can inadvertently affect the immune system’s overall capacity.