NK Cell – Structure, Function & Diseases
NK cell
NK cells are part of the innate immune system and belong to the group of leukocytes , the white blood cells. Their main task is to recognize infected and degenerated endogenous cells and to attack the cells directly with cytotoxic substances that partially dissolve the membrane of the target cell and initiate their programmed cell death. NK cells recognize “normal” body cells by MHC-I structures that healthy cells show on their surface.
What is the NK cell?
NK cells (natural killer cells) are a special type of white blood cell that patrol the blood and lymph . They are part of the innate immune system and recognize healthy, endogenous cells by special structures, the so-called MHC-I molecules (Major Histocompatibility Complex), which are only fully present in healthy cells.
If cells with incomplete MHC-I molecules are identified, they are most likely infected by intracellular microorganisms or degenerated cells ( tumor cells ). The NK cell is then immediately activated and attacks the cell recognized as infected or degenerate. They are able to release cytotoxic substances that cause the target cells to partially dissolve their membranes and trigger apoptosis , programmed cell death.
The counterpart to the NK cells are the T lymphocytes, which are part of the adaptive, acquired immune system. They each specialize in a specific pathogen, which is reflected in additional structures on the cell surface and is referred to as an antigen .
Anatomy & Structure
The NK cells form from lymphatic progenitor cells that originate from the bone marrow . The fully differentiated NK cells are released into the bloodstream and lymphatic system, where they immediately begin their patrols.As a special feature, NK cells have numerous vesicles that contain cytotoxic substances such as perforin for dissolving the membrane of the attacked cell and proteases, which serve for the apoptotic decomposition of the cell and the viral RNA. The apoptosis of the target cell has the advantage that, for example, defined fragments from proteins are produced, down to individual amino acids , which are introduced back into the metabolism . NK cells are characterized by special receptors on their surface that react with the MHC-I structures of the body’s own cells.
These are KIR receptors (killer cell immunoglobulin-like receptors) and so-called natural cytotoxic receptors (NCR). With the KIR receptors, a distinction is made between activating and inhibiting receptors. The NCR are also important for friend-foe recognition and for deciding whether to attack or not.
Function & Tasks
The main task of NK cells is to identify and combat degenerated body cells. Degenerate body cells can be intracellularly infected cells or tumor cells. The NK cells are dependent on their receptor system, which can only check the completeness of their MHC-I structures in the target cells, but not additional structures such as antigens.
Because some viruses use the specific vulnerability of NK cell recognition to evade their “host cell” from the killer system, the NK cells work closely with the cytotoxic T cells, which as state-of-the-art developments in evolution are part of the adaptive, i.e. the acquired immune system , are. However, the T cells are each only specialized for a single antigen, so that a large number of differently specialized T cells are required in order to cover the diverse spectrum of viruses that are possible for an infection.
NK cells could also be referred to as first-line defense cells because they can immediately attack a degenerated cell or a cell infected intracellularly by microorganisms. They are comparable to an armed police force that not only investigates, but can also use armed force to intervene if necessary. Because the NK cells are also tricked by certain intracellular pathogens – especially viruses – the support of the cytotoxic T cells makes sense.
When fighting diseased cells, time can play a major role, for example to forestall an exponential multiplication of viral RNA. The task of the NK cells is therefore to attack the target cell with cytotoxic substances in such a way that the viral RNA is also broken down in order to prevent it from further replication.
Diseases
The immune system is also subject to hormonal influences in all its dynamics. Even sympathetic and parasympathetic system controls influence NK cells and cytotoxic T cells. Interestingly, when the body is sympathetically attuned to acute stress and thus to maximum physical performance, NK cells are also increased and put on “increased alertness”.The cytotoxic T-cells are slowed down by acute stress phases, which was apparently set up by evolution in this way, because rapid immune responses are advantageous during an acute threat with a risk of injury and the associated risk of infection. In the case of chronic stress, the situation is different. Chronic stress leads to a weakening of the immune system, NK cells and T cells not only decrease in number, but also in their alertness. Therefore, high-performance athletes often show an increased susceptibility to infection shortly before major competitions.
Decreased activity of NK cells can also result from undesirable side effects from exposure to drugs ( chemotherapy ) or radiation, while hereditary malfunctions in NK cells are extremely rare. The role of NK cells in tissue-specific autoimmune diseases such as type 1 diabetes mellitus , multiple sclerosis and Hashimoto’s or in systemic autoimmune diseases has not yet been sufficiently clarified. It is conceivable that NK cells in association with T cells have an activating effect on them, so that the T cells carry out the actual attacks on the body’s own cells.
On the other hand, NK cells can also recognize activated, auto-reactive T cells as degenerate and kill them directly. This means that there is a high probability that NK cells can intervene initiating and promoting as well as healing in autoimmune diseases.
Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.