Cells, which then differentiate into participants in an immune response, arise at a very early stage of individual development. Their stem, initial elements appear in the yolk sac, and then enter the liver - the organ of blood formation of the embryo. From the embryonic liver, these cells migrate to the thymus, and after the 4th month the bone marrow remains a reservoir of stem lymphoid cells, which, as necessary, come from it into the bloodstream and develop into mature immune cells.
"Training" in the thymus teaches lymphocytes the ability to distinguish the cells of their body from alien or their cells, but modified. The receptors of immature lymphocytes interact with the surface of the epithelial cells of the thymus, on which there are antigens of tissue compatibility, determining the individuality of the organism. Communication with these receptors stimulates the maturation of lymphocytes within the thymus. In the future, T-lymphocytes will be intolerant only to that foreign antigen, its individual molecules, which are associated with the familiar antigens of tissue compatibility. This determines the focus of the immune response on the complex of his and others. Separately, their antigens (if they are not changed) and others (if they are not cleaved and not combined with their own) do not cause an immune response.Go
T cells evacuated from the thymus can persist in a person for weeks, months, or even years. In response to the action of the antigen, they significantly increase in size, rapidly divide and produce a number of hormone-like protein substances, lymphokines. The latter help mobilize other protective cells and remove dangerous microbes and malfunctioning cells from the body.
The first from a macrophage information about the antigen is obtained by T-cells, called assistants, or T-helpers. The signal for their reproduction is soluble macrophage interleukin-1 (IL-1) factor. It was opened in 1972, Jerry (USA). IL-1 activates inflammatory reactions in the body, in particular, contributes to an increase in body temperature - an adaptive response to inhibit the proliferation of microbes and atypical cells. Under the influence of IL-1, T-helpers begin to produce another hormone-like factor IL-2. Within 6 hours after that, resting T-cells become T-killers, which have a specific cytotoxic effect on tumor target cells or cells infected with microbes. Cytotoxic T-lymphocytes perceive the signal from T-helpers (like T-helpers from macrophages) only if they are compatible with class II antigens (1a-antigens), and T-lymphocytes have a killer effect on antigens compatible with them I class. This very important restrictive mechanism of immunological interactions received in the special literature the name of immunological restriction (restriction, deterrence).
With age, the value of the thymus begins to weaken, since the majority of the cells that have moved out of it are T-reservations in other lymphoid organs and are capable of self-maintenance. But its arming T-cells hormone thymosin thymus produces to a very old age. Any damage to the body (whether emotional stress, depression or mechanical injury) causes a strong reaction from the thymus. The thymus temporarily atrophies, as if it decreases in size, becomes pale, anemic. The temporal involution of the thymus is caused by the adrenal glands, whose cortical substance reacts to stressful effects more quickly. An adaptive reaction during extreme conditions also affects the bone marrow, the thyroid gland, the hypothalamus and the pituitary gland.
In the bone marrow, lymphocytes of other classes arise, including B-lymphocytes, which received their name from Burza, an organ of the birds that regulates the maturation of such cells. The bone marrow is located in the cavities of the tubular and some flat bones, but, despite the anatomical dispersion, it functions as a single organ. Within 1 hour, 200–300 stem lymphoid cells are evicted from it, from which T and B lymphocytes later mature. Under special conditions (stress, microbial attack, radiation, etc.), the rate of maturation of stem cells accelerates. The processes of further specialization of immune cells occur in the spleen, lymph nodes, Peyer's patches of the intestine, pharyngeal tonsils. The central organs of immunity produce soluble mediators perceived on the periphery of T- and B-lymphocytes. Therefore, the thymus and bone marrow are included in the overall organismic system of the endocrine organs.
B cells are somewhat larger than T cells (8.5 and 6.5 μm).