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Organs of the immune system

 
, medical expert
Last reviewed: 31.05.2018
 
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The organs of the hematopoiesis and the immune system are closely related to each other by a common structure, origin and functions. Reticular tissue is the stroma and bone marrow (the organ of the hematopoiesis), and the organs of the immune system. The progenitor of all blood cells and the immune (lymphoid) system in humans are stem cells of the bone marrow, which have the capacity for multiple (up to 100 times) division. In this regard, stem cells constitute a self-sustaining population. Thus, the bone marrow (red) is both an organ of hematopoiesis and an organ of the immune system.

In the bone marrow, precursor cells formed from stem cells are located, which by complex transformations (multiple division) and differentiation along three lines (erythropoiesis, granulopoiesis, thrombocytopoiesis) become the blood constituents: red blood cells, leukocytes, platelets - and enter the bloodstream.

From the stem cells in the bone marrow also develop cells of the immune system - B-lymphocytes, and of the latter - plasma cells (plasmocytes). Part of the stem cells from the bone marrow enters the bloodstream and then enters another central organ of the immune system - the thymus (thymus gland), where they also give rise to immunocompetent cells - T-lymphocytes.

In the population of T-lymphocytes, several subpopulations are isolated: T-helpers, delayed-type hypersensitivity T-effectors (GZT), T-killers (T-effectors of cytotoxic reactions), T suppressors.

T-helpers activate B-lymphocytes, they include them in the formation of antibodies.

T-effectors of HRT involve other cells (the monocytes that differentiate into macrophages) into the immune process, interact with granulocytes (basophilic and eosinophilic leukocytes) and also involve them in the response of the immune response.

T-killers destroy foreign target cells, such as tumor cells, mutant cells; participate in reactions of rejection of transplanted tissues, in antiviral immunity.

T-suppressors suppress the activity (function) of T and B cells (T and B lymphocytes).

B-lymphocytes finally mature in the bone marrow. Part of B-lymphocytes (antigen-dependent) is activated after their contact with the antigen.

In reactions of the immune response, T- and B-lymphocytes participate in a friendly manner, form various models of lymphocyte interactions.

The immune system unites organs and tissues that protect the body from genetically alien cells or substances coming from outside or formed in the body itself.

The organs of the immune system containing the lymphoid tissue perform the function of "protecting the constancy of the internal environment of the organism throughout the life of the individual". They produce immunocompetent cells, primarily lymphocytes, as well as plasma cells, incorporate them into the immune process, ensure the recognition and destruction of foreign cells and substances "that bear the signs of genetically alien information" that have penetrated into the body or formed in it. Genetic control is performed by jointly functioning populations of T and B lymphocytes, which, with the participation of macrophages, provide an immune response in the body.

The immune system, according to modern data, is made up of all organs that participate in the formation of cells of the lymphoid series, perform protective reactions of the organism, create immunity-immunity to substances possessing foreign antigenic properties. Parenchyma of all organs of the immune system is formed by lymphoid tissue, which consists of two components - the reticular stroma and lymphoid cells. The reticular stroma is formed by the reticular cells and fibers that form a shallow network. In the loops of this network are lymphocytes of varying degrees of maturity, plasmocytes, macrophages and other associated cells.

The bone marrow, in which the lymphoid tissue is closely related to the hematopoietic, thymus, lymph nodes, spleen, accumulation of lymphoid tissue in the walls of the hollow organs of the digestive, respiratory and urinary tracts (tonsils, lymphoid plaques, single lymphoid nodules) belong to the organs of the immune system. These organs are also called lymphoid organs, or organs of immunogenesis.

Depending on the function and position in the human body, the organs of the immune system are divided into central and peripheral. The central organs of the immune system include the bone marrow and thymus. In the bone marrow of its stem cells B-lymphocytes (bursodependents) are formed, independent in their differentiation from the thymus. In the human immunogenesis system, the bone marrow is now considered as an analogue of Fabricius's bag (bursa) - a cellular cluster in the cloacal wall of the intestine in birds. In the thymus, there is a differentiation of T-lymphocytes (thymus-dependent) formed from the stem cells of the bone marrow that have entered this body. Subsequently, B- and T-lymphocytes with blood flow enter the peripheral organs of the immune system, which include tonsils, lymphoid nodules located in the walls of the hollow organs of the digestive and respiratory systems, the urinary system, lymphoid plaques in the walls of the small intestine, lymph nodes and spleen , as well as numerous lymphocytes that freely move in organs and tissues in order to search for, recognize and destroy foreign substances.

T-lymphocytes colonize the thymus-dependent (paracortical) zone of the lymph nodes, the spleen (periarterial lymphoid clutches and the periarterial lymphoid nodules) and ensure the implementation of cellular immunity by accumulation and activation of sensitized (with increased sensitivity) lymphocytes, as well as humoral immunity (by synthesizing specific antibodies).

B-lymphocytes are precursors of antibodies of the forming cells-plasmocytes and lymphocytes with increased activity. They enter bursoid-dependent zones of lymph nodes (lymphoid nodules and flesh strands) and spleen (lymphoid nodules, except for their periarterial part). B-lymphocytes perform the function of humoral immunity, in which the main role belongs to blood, lymph, the secret of glands containing substances (antibodies) involved in immune reactions.

T- and B-lymphocytes can not be distinguished from each other in a light microscope. Under a large increase in the electron microscope with a scanning attachment on the surface of B-lymphocytes, numerous microvilli are seen. On these microvilli, the molecular sizes of the structure are located-receptors (sensory apparatuses) that recognize antigens-complex substances that cause an immune response in the body. This reaction consists in the formation of antibodies by cells of the lymphoid series. The number (density of location) of such receptors on the surface of B-lymphocytes is very high. Cells that perform an immune response are also called immunocompetent cells (immunocytes).

The central organs of the immune system are located in well protected areas: bone marrow - in the medullary cavity, thymus - in the thoracic cavity behind the sternum.

Peripheral organs of the immune system are located on the borders of habitats of microflora, in areas of possible introduction into the body of foreign substances. Here, as it were, border guard zones are formed - "guard posts", "filters" containing lymphoid tissue. Tonsils lie in the walls of the initial section of the digestive tube and respiratory tract, forming the so-called pharyngeal lymphoid ring (Pirogov-Valdeier ring). The lymphoid tissue of tonsils is present on the border of the oral cavity, the nasal cavity - on one side and the pharynx and laryngeal cavity - on the other. Lymphoid (Peyer's) plaques are located in the walls of the small intestine, mainly the ileum, near the point of its entry into the blind, near the border of two different sections of the digestive tube: the small and large intestines. On the other side of the ilio-cecal valve numerous lymphoid nodules densely lying next to each other are in the walls of the appendix. Single lymphoid nodules are scattered in the thickness of the mucous membrane of the digestive organs, respiratory and urinary tracts for the implementation of immune surveillance at the border of the body and the external environment, represented by air, the contents of the digestive tract, excreted from the body by urine.

Numerous lymph nodes lie on the lymphatic pathways (tissue fluid) from organs and tissues to the venous system. The foreign agent that enters the lymph flow from the tissue fluid is retained in the lymph nodes and is rendered harmless. On the path of blood flow from the arterial system (from the aorta) to the portal vein system, branching in the liver, lies the spleen, whose function is the immune control of the blood.

The characteristic morphological signs of the immune system organs are early laying (in embryogenesis) and the state of maturity already in newborns, as well as significant development in childhood and adolescence, i.e. In the period of formation and maturation of the organism and the formation of its protective systems. In the future, the age-related involution of both the central and peripheral organs of the immune system occurs quite rapidly. They are quite early (starting with adolescence and adolescence), the amount of lymphoid tissue decreases, and its place is occupied by a growing connective (fatty) tissue.

Lymphoid tissue of the immune system is characterized by the presence of lymphoid nodules, both without a breeding center, and with such a center (the center of cell division and the formation of new lymphocytes).

The total mass of organs of the immune system in the human body is (without bone marrow) about 1.5-2.0 kg (approximately 10 12 lymphoid cells).

trusted-source[1], [2], [3], [4], [5]

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