Related articles: antibodies and immunoglobulins
Lymphocytes are cells of the human body responsible for "acquired immunity. This last term emphasizes the ability of the immune system to selectively fight - through hyperspecialized cells, called lymphocytes - every different antigen that attacks it. At the first immunological exposure the response times are rather long, but thanks to the preservation of a "memory" the subsequent attacks are eradicated much more quickly and effectively. It is on this principle that vaccinations are based.
Only 5% of the body's lymphocytic stock is present in the bloodstream; the preponderant amount of lymphocytes is instead found in the lymphatic tissues (spleen, thymus, and above all lymph nodes). At this level, the lymphocytes have the ability to mature and act. readily against antigens that have penetrated into the organism through mucous membranes or continuous solutions of the skin. In the presence of a severe infection, the lymphocytes multiply rapidly, increasing - sometimes considerably - the volume of the lymph nodes.
There are three types of lymphocytes: B lymphocytes, T lymphocytes, and natural killer cells. We have already talked about the former in the article dedicated to immunoglobulins (antibodies): B lymphocytes can be compared to many sentinels, each of which has a small number of clones capable of recognizing a specific antigen thanks to the presence of receptors (antibodies) on the own external membrane. When during the long wandering in the blood, a B lymphocyte encounters its own antigen, it proliferates several times giving rise to daughter cells called clones; a part of the clonal population is activated in plasma cells, which synthesize in large quantities the specific antibodies present on the membrane of their precursor; the remainder acts as a memory reservoir against future infections, which will be countered more quickly and effectively. The production of lymphocyte clones occurs under the stimulus of T helper lymphocytes (as we will see later). produced by plasma cells, also known as immunoglobulins, bind to s ignoring the danger to the cells responsible for their destruction.
Since B cell-mediated immunity takes advantage of antibodies present in the blood and other humors of the body, it is called humoral immunity.
For what has been said, antibodies are effective only against extracellular pathogens, while if an antigen - for example a virus - is confined inside a cell, the encounter with the B lymphocyte cannot take place. In place of immunoglobulins, then T lymphocytes intervene, capable of recognizing and destroying infected cells, preventing the reproduction of the pathogen and mad cells (tumor). The way in which T lymphocytes recognize abnormal cells and spare the healthy ones is quite complex, delicate and typical of each individual (just think of the phenomenon of rejection in transplants). Let's briefly say that the signal is given by markers, called MHC (or major histocompatibility complex), which contain fragments of antigen recognized as foreign by the T lymphocyte receptors. The difference with the B line receptors (called antibodies or immunoglobulins) is that while the latter are able to bind directly to the antigens, the T cells bind only to other human cells that have fragments of antigen, either because they are infected (MHC class I), or because they are responsible for the digestion of the stranger (MHC In the first case, the intervention of cytotoxic T lymphocytes leads to the destruction of the infected cell, in the second the intervention of T helper lymphocytes increases the immune response.
The T lymphocyte population is not homogeneous, but has several subpopulations:
- Tc lymphocytes (cytotoxic or T killer): lysate target cells leading them to death and favor the action of phagocytes (macrophages);
- Th lymphocytes (T helper, T4 or CD4): they stimulate and support the recognition and response action of T and B lymphocytes (favoring their differentiation into plasma cells and the production of antibodies); they are the elective target of the T and B lymphocytes. "AIDS (HIV);
- Ts lymphocytes (T suppressor): block the activity of helper and cytotoxic T lymphocytes;
- DHT T lymphocytes (T Delayed Type Hypersensitivity): they are mediators of inflammatory phenomena and in particular of delayed hypersensitivity.
Unlike B lymphocytes, whose average lifespan is a few days (except for "memory cells"), T lymphocytes survive for several months or years.
They are responsible for cell mediated immunity (active against intracellular pathogens, such as viruses and some bacteria).
They are responsible for the humoral antibody immunity (active against extracellular antigens).
They are comparable to myriads of sentinels, organized into small families and specialized in recognizing a specific enemy. When they encounter it, they differentiate into plasma cells, which produce antibodies in high quantities. These, also known as immunoglobulins, bind to the target cell signaling it as hostile; this is followed by the intervention of other cells responsible for eliminating the intruder.
Once the antigen has been eradicated, a small population of memory cells remains which, thanks to their long life, are ready to respond to any subsequent exposure to the same antigen.
They determine the rejection of transplants and have an "antitumor action."
They consist of four lymphocyte subpopulations (cytotoxic T lymphocytes, helper T lymphocytes, suppressor T lymphocytes and DHT T lymphocytes), each with specific action.
They recognize and exhibit many surface markers, which play an important role in cell recognition and cooperation.
They intervene at various levels by stimulating and coordinating the immune response; with some of their subpopulations they participate directly in the elimination of the intracellular antigen.
Lymphocytes Natural Killer involved in the early immune response. Their name clearly reveals their biological function: natural killer lymphocytes, in fact, induce the target cell to commit suicide (in particular those cancer infected with viruses). At the same time, they secrete various antiviral cytokines, which induce cells not yet infected to implement mechanisms capable of inhibiting the replication of viruses.
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