Killer cells are part of the immune system. As so-called cytotoxic T cells (acquired immune system) or as natural killer cells (innate immune system), they recognize and attack cells foreign to the body and altered cells belonging to the body, such as cancer cells, cells infected by viruses or bacteria, or aging cells. The killer cells release substances that partially perforate the cell membrane of the attacked cells, causing them to undergo programmed cell death, or apoptosis.
What is a killer cell?
Killer cells are an important part of the immune system. They recognize structures foreign to the body and altered cells belonging to the body, such as cells infected by viruses or bacteria and cells that have degenerated into cancer cells. Two different types of killer cells can be identified, the so-called natural killer cells (NK cells), which are part of the innate immune system, and the cytotoxic T cells, which are part of the adaptive or acquired immune system. To distinguish friend from foe, the two cell types operate with different systems. NK cells have specific receptors located in their plasma membrane that interact with so-called MHC-I (Major Histocompatibility Complex) molecules that healthy endogenous cells display on their surface. If the MHC-I molecules are not present or if certain molecules are missing – as is usually the case with cancer cells or cells infected by viruses – they are activated. While the NK cells work non-specifically, the cytotoxic T cells are characterized by extreme specificity. In infected somatic cells, the MHC-I complexes also display additional peptides or other specific substances, so-called antigens. Cytotoxic T cells are each specialized to recognize only one specific antigen.
Anatomy and structure
NK cells find their origin in lymphoid progenitor cells that develop in the bone marrow and, after differentiation, are released into the blood and lymph channels. As a weapon against cells to be killed, there are numerous lysosomes in their cytoplasm, which are ejected when the NK cell is activated, releasing the cytotoxic substance found in the lysosomes and lysing the target cell. An important anatomical feature is two different types of receptors on their surface. They are inhibitory and activating receptors that react with the MHC-I molecules, present the target cells on their surface and activate or inactivate the NK cells. Cytotoxic T cells also originate from the bone marrow, but take the detour via the thymus for their differentiation, which has also earned them the name T cell. In the thymus, the cells differentiate into T cells and receive their specific T cell receptor before they are also released into the bloodstream. Their specific receptor consists of a protein complex that they carry on their surface and recognize specific antigens that are presented to target cells along with MHC-I molecules.
Function and Tasks
The main task of killer cells is to identify and immediately kill cells infected with viruses or other intracellular pathogens and degenerate tumor cells. The two different types of killer cells, the NK cells and the cytotoxic T cells, are available to perform this task. The evolutionarily much older NK cells have the ability to check the “IDs” of the target cells, their MHC-I molecules for presence and for completeness. If the NK cells encounter cells with incomplete MHC-I molecules or cells without recognizable MHC-I molecules, the NK cells immediately attack. They release substances that lyse the cell membrane of the attacked cells. Apoptosis is usually triggered in the attacked cell, programmed cell death, which involves a kind of self-dissection with defined fragments, most of which are reintroduced into the intermediate metabolism. Macrophages then phagocytose the remnants and transport them away. The evolutionarily much more “modern” cytotoxic killer cells are specialized via their specific receptors only to one specific antigen each, so they do not recognize other antigens, but they have more possibilities in case of their activation. They can mature into T helper cells or cytotoxic T cells at high speed and become active accordingly.They secrete perforins to lyse the membrane of the target cell and granzymes that induce apoptosis. In addition, they secrete interleukins and interferons, regulatory peptides to control the immune response to a viral infection that has occurred. Since a cytotoxic T cell can only recognize “its” specific antigen, the thymus must produce cytotoxic T cells for each type of antigen, of which there are probably several million. The advantage of specialization is that the immune defense can adapt to new requirements, e.g. to continuously modified viruses. In effect, there is a constant race between the adaptive immune system and the genetically changing viruses. To avoid having to constantly maintain a large reservoir of every T cell ever needed, the thymus produces long-lived memory cells that serve as the basis for fighting a new infection with the corresponding pathogen, making the immune response 100 times faster.
Diseases
The work of killer cells is highly dynamic, subject to hormonal control. For example, an acute stress event leads to increased proliferation of NK cells and heightened vigilance, or red alert, as it were. The specifically effective cytotoxic T cells are slowed down because they would be unlikely to make a helpful contribution during an acute situation requiring rapid responses. During chronic stress, on the other hand, the entire immune system is weakened. All types of killer cells decrease in number and vigilance, increasing susceptibility to infection. One of the most important diseases associated with cytotoxic T cells are autoimmune diseases, in which the killer cells do not recognize the body’s own cells as such, but attack them and form corresponding autoimmune antibodies. The mechanism for the development of autoimmune diseases is not yet fully understood. It is generally accepted that genetic factors play at least a favoring role.
