Natural Killer (NK) cells are a specialized type of white blood cell that plays a significant role in the body’s immediate defense. As part of the innate immune system, they act as the body’s first line of defense against various threats. NK cells respond swiftly, offering rapid protection without needing prior exposure to specific pathogens or abnormal cells. Their quick action makes them important for immune surveillance.
What Are Natural Killer Cells?
Natural Killer cells are a unique type of lymphocyte, which also includes T and B cells. Unlike T and B cells, which belong to the adaptive immune system and develop specific recognition after exposure, NK cells are part of the innate immune system and do not require prior sensitization. They are often described as “first responders” due to their immediate action.
These cells originate and mature in the bone marrow. From there, they circulate throughout the body, with significant populations found in the peripheral blood, lymph nodes, and spleen. NK cells represent approximately 5–20% of all circulating lymphocytes. They are characterized by the presence of CD56 and the absence of CD3 markers on their surface, distinguishing them from other lymphocyte types.
How Natural Killer Cells Function
Natural Killer cells identify and eliminate compromised cells through a concept known as “missing self” recognition. Healthy cells display specific surface markers, primarily Major Histocompatibility Complex class I (MHC-I) molecules, which act as “self” identifiers. NK cells possess inhibitory receptors that bind to these MHC-I molecules, signaling them not to attack healthy cells.
However, cells that are infected by viruses or have become cancerous often reduce or lose the expression of their MHC-I markers, a strategy to evade detection by other immune cells like cytotoxic T cells. This “missing self” signal, or the absence of sufficient inhibitory signals, prompts the NK cell to activate. Concurrently, stressed or abnormal cells may also upregulate activating ligands, further tipping the balance towards NK cell activation.
Once activated, NK cells directly eliminate target cells through a process called cytotoxicity. They achieve this by releasing specialized cytotoxic granules containing proteins such as perforin and granzymes. Perforin creates pores in the membrane of the target cell. These pores allow granzymes, which are proteolytic enzymes, to enter the compromised cell. Inside the target cell, granzymes initiate a cascade of events that lead to apoptosis, or programmed cell death.
Natural Killer Cells and Disease Protection
Natural Killer cells are effective in defending the body against two major categories of threats: viral infections and cancer. Their ability to recognize and destroy compromised cells without prior exposure makes them an immediate defense.
For viral infections, NK cells act as an early line of defense. Viruses often manipulate infected cells to downregulate MHC-I expression, making them invisible to T cells. NK cells, however, specifically target these cells, preventing viral replication and spread. They are especially important against common viral infections like influenza and herpesviruses.
Beyond viral defense, NK cells play a role in anti-cancer immunity. They identify and eliminate tumor cells, even in the absence of specific surface antigens that T cells typically require for recognition. Cancerous cells, much like virally infected cells, can alter their surface markers, including reducing MHC-I expression, to escape detection by the adaptive immune system. NK cells sense these alterations and initiate a cytotoxic response, contributing to the body’s continuous surveillance against tumor development and progression. Patients with lower NK cell activity or deficiencies have increased risks of cancer and severe viral infections.
Supporting Healthy Natural Killer Cell Activity
Maintaining healthy Natural Killer cell activity is influenced by lifestyle factors. Adequate sleep is beneficial for immune function, and studies suggest that sleeping less than 7 hours per night reduces NK cell activity. Aiming for 7-9 hours of sleep supports the immune system, including NK cells.
Regular moderate exercise, such as brisk walking for 30 to 60 minutes daily, positively affects NK cell activity. While intense, prolonged exercise might temporarily suppress NK cell function, consistent moderate activity generally enhances their cytolytic capabilities. Managing stress through techniques like meditation or yoga is also helpful, as chronic stress can suppress NK cell activity. A balanced diet rich in fruits, vegetables, and whole grains, along with probiotic-rich foods like yogurt and kefir, further supports NK cell function by providing essential nutrients and promoting a healthy gut microbiome.