Immune system organs

The organs of hematopoiesis and the immune system are closely related by their common structure, origin and functions. Reticular tissue is the stroma of both the bone marrow (the organ of hematopoiesis) and the organs of the immune system. The progenitor of all blood cells and the immune (lymphoid) system in humans are the stem cells of the bone marrow, which have the ability to divide multiple times (up to 100 times). In this regard, stem cells form a self-sustaining population. Thus, the bone marrow (red) is simultaneously both the organ of hematopoiesis and the organ of the immune system.

The bone marrow contains precursor cells formed from stem cells, which, through complex transformations (multiple division) and differentiation along three lines (erythropoiesis, granulopoiesis, thrombocytopoiesis), become formed elements of the blood: erythrocytes, leukocytes, thrombocytes - and enter the bloodstream.

Stem cells in the bone marrow also give rise to immune system cells - B-lymphocytes, and from the latter - plasma cells (plasmocytes). Some of the stem cells from the bone marrow enter the blood, and then enter another central organ of the immune system - the thymus (thymus gland), where they also give rise to immunocompetent cells - T-lymphocytes.

In the T-lymphocyte population, several subpopulations are distinguished: T-helpers, T-effectors of delayed-type hypersensitivity (DTH), T-killers (T-effectors of cytotoxic reactions), T-suppressors.

T-helpers activate B-lymphocytes and include them in the process of antibody formation.

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

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

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

B-lymphocytes finally mature in the bone marrow. Some B-lymphocytes (antigen-dependent) are activated after their contact with the antigen.

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

The immune system unites organs and tissues that provide protection to the body from genetically foreign cells or substances coming from outside or formed in the body itself.

The organs of the immune system containing lymphoid tissue perform the function of "protecting the constancy of the internal environment of the body throughout the life of the individual." They produce immunocompetent cells, primarily lymphocytes, as well as plasma cells, include them in the immune process, and ensure the recognition and destruction of foreign cells and substances that have entered the body or formed in it, "bearing signs of genetically foreign information." Genetic control is carried out 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, consists of all organs that participate in the formation of lymphoid cells, carry out protective reactions of the body, create immunity - insensitivity to substances with foreign antigenic properties. The parenchyma of all organs of the immune system is formed by lymphoid tissue, which consists of two components - reticular stroma and lymphoid cells. The reticular stroma is formed by reticular cells and fibers that form a fine-meshed network. Lymphocytes of varying degrees of maturity, plasma cells, macrophages and other accompanying cells are located in the loops of this network.

The organs of the immune system include the bone marrow, in which lymphoid tissue is closely associated with hematopoietic tissue, the thymus, lymph nodes, spleen, and accumulations of lymphoid tissue in the walls of hollow organs of the digestive, respiratory, and urinary tracts (tonsils, lymphoid plaques, and solitary lymphoid nodules). These organs are also called lymphoid organs or organs of immunogenesis.

Depending on their 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, B-lymphocytes (bursa-dependent) are formed from its stem cells, independent in their differentiation from the thymus. In the human immunogenesis system, bone marrow is currently considered an analogue of the bursa of Fabricius - a cellular accumulation in the wall of the cloacal section of the intestine in birds. In the thymus, differentiation of T-lymphocytes (thymus-dependent) occurs, formed from the stem cells of the bone marrow that enter this organ. Subsequently, B- and T-lymphocytes enter the peripheral organs of the immune system with the blood flow, which include the 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 move freely in organs and tissues in order to search for, recognize and destroy foreign substances.

T-lymphocytes populate the thymus-dependent (paracortical) zone of the lymph nodes, spleen (periarterial lymphoid cuffs and periarterial part of the lymphoid nodules) and ensure the implementation of cellular immunity by accumulating and activating sensitized (with increased sensitivity) lymphocytes, as well as humoral immunity (by synthesizing specific antibodies).

B-lymphocytes are precursors of antibody-forming cells - plasma cells and lymphocytes with increased activity. They enter the bursa-dependent zones of the lymph nodes (lymphoid nodules and pulpy cords) and the spleen (lymphoid nodules, except for their periarterial part). B-lymphocytes perform the function of humoral immunity, in which the main role belongs to the blood, lymph, and glandular secretion containing substances (antibodies) participating in immune reactions.

It is impossible to distinguish T- and B-lymphocytes from each other in a light microscope. Under high magnification of an electron microscope with a scanning attachment, numerous microvilli are visible on the surface of B-lymphocytes. On these microvilli are molecular-sized structures - receptors (sensitive devices) that recognize antigens - complex substances that cause an immune reaction in the body. This reaction consists of the formation of antibodies by lymphoid cells. The number (density of arrangement) of such receptors on the surface of B-lymphocytes is very high. Cells that perform an immune reaction are also called immunocompetent cells (immunocytes).

The central organs of the immune system are located in well-protected places: the bone marrow is in the medullary cavities, the thymus is in the chest cavity behind the manubrium of the sternum.

The peripheral organs of the immune system are located at the boundaries of microflora habitats, in areas where foreign substances can enter the body. Here, as it were, border, protective zones are formed - "guard posts", "filters" containing lymphoid tissue. The tonsils are located in the walls of the initial section of the digestive tract and respiratory tract, forming the so-called pharyngeal lymphoid ring (Pirogov-Waldeyer ring). The lymphoid tissue of the tonsils is located at the border of the oral cavity, nasal cavity - on one side and the cavity of the pharynx and larynx - on the other. Lymphoid (Peyer's) plaques are located in the walls of the small intestine, mainly the ileum, near the place where it flows into the cecum, near the border of two different sections of the digestive tract: the small and large intestines. On the other side of the ileocecal valve, numerous lymphoid nodules lying tightly next to each other are located in the walls of the appendix. Single lymphoid nodules are scattered in the thickness of the mucous membrane of the digestive, respiratory and urinary tracts to carry out immune surveillance at the border of the body and the external environment, represented by air, the contents of the digestive tract, and urine excreted from the body.

Numerous lymph nodes lie on the paths of lymph (tissue fluid) from organs and tissues to the venous system. A foreign agent entering the lymph flow from the tissue fluid is retained in the lymph nodes and rendered harmless. The spleen lies on the path of blood flow from the arterial system (from the aorta) to the portal vein system, branching out in the liver. Its function is immune control of the blood.

Characteristic morphological features of the immune system organs are early formation (in embryogenesis) and maturity in newborns, as well as significant development in childhood and adolescence, i.e. during the period of formation and maturation of the organism and the formation of its defense systems. Later, age-related involution of both central and peripheral organs of the immune system occurs quite quickly. In them, the amount of lymphoid tissue decreases quite early (starting from adolescence and youth), and its place is taken by growing connective (fatty) tissue.

The lymphoid tissue of the organs of the immune system is characterized by the presence of lymphoid nodules both without a reproduction center and with such a center (a center for cell division and the formation of new lymphocytes).

The total mass of the immune system organs in the human body (excluding bone marrow) is about 1.5-2.0 kg (approximately 10 ¹² lymphoid cells).

[ 1 ], [ 2 ], [ 3 ], [ 4 ]