1. Recognizing Infected or Abnormal Cells:
* Missing MHC I: NK cells detect the presence of infected or cancerous cells by looking for the absence of major histocompatibility complex class I (MHC I) proteins on the surface of cells. MHC I molecules normally present fragments of proteins found inside a cell, allowing the immune system to identify its own cells. Infected or cancerous cells often downregulate MHC I expression, signaling to NK cells that something is wrong.
* Stress Signals: NK cells also recognize stress signals released by infected or cancerous cells, such as altered cell surface molecules or the production of specific cytokines (signaling proteins).
2. Initiating Attack:
* Direct Killing: Once NK cells identify a target, they directly kill the infected or cancerous cell through two main mechanisms:
* Perforin and Granzyme Release: NK cells release perforin, a protein that forms pores in the target cell's membrane. Then, they release granzyme, a family of enzymes that enter the cell through the pores and activate programmed cell death (apoptosis).
* Death Receptor Signaling: NK cells can also directly trigger apoptosis in target cells by interacting with death receptors on their surface.
* Cytokine Production: NK cells also secrete cytokines, such as interferon-gamma (IFN-γ) and tumor necrosis factor-alpha (TNF-α), which help to activate other immune cells, enhance inflammation, and further control the infection.
3. Role in Immune Response:
* Early Defense: NK cells are one of the first responders to infection, acting quickly to control the spread of pathogens before the adaptive immune system (T cells and B cells) can mount a specific response.
* Tumor Surveillance: NK cells play a crucial role in the immune surveillance of tumors, preventing the growth and spread of cancer cells.
* Immune Regulation: NK cells can also help to regulate the adaptive immune response, ensuring that it is appropriately targeted and does not become excessive.
Overall, NK cells are essential for maintaining immune homeostasis and protecting the body from various threats. They act as a crucial first line of defense against infection and can contribute to both innate and adaptive immunity.
