Introduction
Natural Killer cells are a vital part of the body’s innate immune system, acting as the first line of defense against infections and cancer. Unlike other immune cells that require prior sensitization, NK cells can recognize and destroy infected or abnormal cells without prior exposure. Their unique ability to target cells lacking “self” markers makes them essential in preventing the spread of diseases. NK cell research is rapidly expanding, with promising applications in immunotherapy and advanced cancer treatments.
What Are Natural Killer Cells?
Natural Killer (NK) cells are a specialized type of white blood cell belonging to the innate immune system. They are unique in their ability to identify and destroy abnormal cells, such as virus-infected cells or cancer cells, without prior sensitization. Unlike T cells and B cells, NK cells do not rely on specific antigens to trigger their response. Instead, they detect changes in the surface molecules of potential target cells, especially the absence or alteration of major histocompatibility complex (MHC) class I molecules.
Characteristics of NK Cells
Belong to the lymphoid lineage but lack antigen-specific receptors like T or B cells.
Possess cytotoxic granules containing perforin and granzymes. Can recognize stressed or abnormal cells without MHC restriction. Express both activating and inhibitory receptors to regulate function. Short lifespan but highly potent in immune surveillance. Function independently of prior antigen exposure.
Functions of NK Cells
Destroy virus-infected cells through direct cytotoxicity.
Target and kill tumor cells without prior sensitization.
Release cytokines such as IFN-γ to activate other immune cells.
Regulate immune responses by interacting with dendritic cells, macrophages, and T cells.
Contribute to tissue remodeling and repair in certain conditions.
Maintain balance between immune activation and tolerance.
Activation Mechanisms
Triggered by absence or alteration of MHC class I molecules ("missing-self" recognition).
Activated through binding to stressinduced ligands on target cells.
Receive signals from cytokines like IL-2, IL12, and IL-15.
Modulated by a balance between activating (e.g., NKG2D) and inhibitory receptors (e.g., KIRs).
Enhanced activity in presence of antibody-coated targets via antibodydependent cellular cytotoxicity (ADCC).