Our body is constantly being confronted with pathogens. Thanks to the immune system, such collisions rarely lead to the development of diseases. Lymphocytes are directly involved in the recognition and destruction of pathogens. In this topic, we will analyze the classification of lymphocytes and the structure of individual cell types that help them perform their specific functions.
Lymphocytes
All lymphocytes contribute to the immune system's protection of our body, but each individual type of cell performs this function in different ways. There are three main types of lymphocytes: B cells, T cells, and NK cells (natural killer cells).
Lymphocytes are largely responsible for the development of adaptive immunity — immunity intended to develop and remember pathogen-specific protection. Adaptive immunity is highly specific because it is directed against individual foreign molecules — antigens. Adaptive immune cells acting on the pathogen are not initially found in the body, but are formed in the process of an immune response to an antigen when it first enters the body. This is the main difference between adaptive immunity and innate immunity. As a result of the adaptive immune response, immunological memory is formed, which gives a stronger and faster response upon repeated encounter with the same antigen. As an example, we can cite a common disease — chicken pox. Once a person has had chicken pox, they have an almost 0% chance of getting sick with it again because memory cells are formed after disease, and they help adaptive immunity to instantly respond to the virus. Adaptive immunity "learns" throughout life and is based on high specificity for pathogen molecules; for this, the body needs various types of different cells.
Lymphopoiesis
All three types of lymphocytes develop from a single type of progenitor cell, the prolymphocyte. Prolymphocytes mature into lymphocytes in the bone marrow during a process called lymphopoiesis, during which they are subdivided into three main lines — B, T, and NK cells. In the process of maturation, migration occurs: T cells migrate to the thymus gland and B cells migrate to the lymph glands. Once in the thymus, young lymphocytes undergo selection — a test of the adequacy of the immune response. This selection is critical, not only to more effectively kill pathogens, but also to ensure that T-killers do not kill healthy cells in the body, causing autoimmune disease.
T-lymphocytes
T cells are the largest group of lymphocytes, and can be further divided into three main subgroups: T-killers, T-helpers, and T-suppressors. T-killers, as the name implies, are engaged in the destruction of pathogens, particularly viruses. Their main targets are infected cells, which must be destroyed along with the pathogen inside them. T-killers have special granules containing the proteins of the granzyme-perforin system [pic 2]. Upon activation of T-cells, these proteins are released and act on the target cell. Perforin perforates the cell membrane, and granzymes enter the target cell through the pores and enzymatically trigger a cascade of reactions leading to apoptosis — cell death. T-killers develop in the thymus, or thymus gland.
T-helpers cells are, well, helper cells. They participate in the activation of killers, but also affect the activity of other lymphocytes and even phagocytes. T-helpers also produce a large number of different factors, such as interleukins (specific proteins in the blood), which modulate the development of the immune process. Like T-killer cells, T-helpers also undergo selection.
T-suppressors complete the immune response. They secrete specific molecules that suppress the immune response so it doesn't go too far and damage the body it is supposed to protect.
B-lymphocytes
B-lymphocytes that have just entered the bloodstream from the bone marrow are called naive lymphocytes. To become a mature lymphocyte, it needs contact with an antigen — a pathogen molecule — to start the process. Several days later it will have transformed into a plasma cell.
A mature B-lymphocyte acts as an antigen-presenting cell and is a source of specific molecular substances for the regulation of the immune response. However, its main task is to produce antibodies. Antibodies (or immunoglobulins) are large, Y-shaped, globular proteins that neutralize the molecular and cellular structures of pathogens. The next time this pathogen enters the body, the antibodies will be ready. Subsequent meetings with the antigen often go unnoticed by the body, so the immune response develops quickly and effectively. The receptor gene rearrangement system exists to increase specificity of antibodies. The genes encoding antibodies are highly polymorphic. In the process of maturation of naive lymphocytes that have not yet encountered a pathogen, the genes encoding antigen-recognizing receptors are rearranged to form many different antibodies.
Some B-lymphocytes turn into memory cells to "remember" the pathogen and its antigens. When a memory cell encounters the same antigen again, it will be able to transform into a plasma cell more quickly than the first time and immediately secrete a large amount of antibodies.
NK-cells
The third group of lymphocytes are natural killer cells. They differ from B- and T-lymphocytes in their functions. Natural killers are not responsible for the immune system activation or recognition. They destroy cells within the body as part of a "self-cleansing" system. Most often, they eliminate cells that are dividing when they shouldn't be. This is important because unregulated cell proliferation can provoke the development of a tumor. NK cells also eliminate cells infected with pathogens, in particular viruses (these are related to T-cells).
NK cells recognize infected cells without the participation of MHC and antibodies, so their response is much faster. NK cells contain cytoplasmic granules with proteins of the granzyme-perforin system like T-killers. NK cells also have a kind of memory cell that allow them to increase their population dramatically if a pathogen is detected.
Conclusion
Lymphocytes are a large group of cells formed from a single precursor cell. Lymphocytes can be divided into three groups: B cells, which either "remember" a pathogen or synthesize antibodies; T cells, which destroy infected cells and help regulate the immune process; and NK cells, which destroy infected cells and tumor cells. Together, they form the body's immune response.