Natural killer (NK) cells are a type of lymphocyte that play a crucial role in the immune system, specifically within the realm of innate immunity. Unlike T cells and B cells, which are components of adaptive immunity and exhibit specificity in antigen recognition, NK cells are characterized by their lack of specificity. This means they can respond to a wide range of threats without prior sensitization to specific antigens.
The primary function of NK cells is to identify and eliminate infected host cells. They achieve this by recognizing irregular patterns on the surface of these cells. When an NK cell detects an infected cell, it releases cytotoxic molecules known as death packages. These packages contain perforin, which forms pores in the membrane of the target cell, and protease-containing granules, which are enzymes that degrade proteins. This process can lead to apoptosis, or programmed cell death, of the infected host cell.
NK cells can recognize two main irregular patterns on infected cells. The first is the absence of MHC class I molecules on the cell surface. Some viruses have evolved mechanisms to downregulate MHC class I expression, making the infected cells invisible to T cells, which rely on these molecules for recognition. However, NK cells can still detect these cells lacking MHC class I and initiate apoptosis.
The second pattern recognized by NK cells involves the binding of IgG antibodies to the surface of infected cells. This process is known as antibody-dependent cellular cytotoxicity (ADCC). In this scenario, NK cells bind to the Fc region of the IgG antibodies that are attached to the infected cell. This interaction triggers the NK cells to release their death packages, leading to the destruction of the target cell.
In summary, natural killer cells are essential components of the innate immune response, capable of targeting and eliminating infected cells through mechanisms that do not require prior exposure to specific antigens. Their ability to recognize cells lacking MHC class I and to engage in ADCC allows them to work in concert with the adaptive immune system, enhancing the overall effectiveness of the immune response.
