We often operate with such words as "good immunity". But what is it? What organs and cells form immunity, what does it depend on and what is it all about? In this topic, we will discuss the basics of immunology, the main tissues and organs involved in its formation, and the principles by which the body's immune system operates.
Basics of immunology
Immunology is a science that studies the process of formation of immunity, as well as the molecular and cellular mechanisms of the body's response to pathogens (they are also called antigens). In general, immunity can be divided into two parts – innate immunity and acquired immunity. Innate immunity does not need to be "taught", while acquired immunity cells are constantly learning, and the organism has a whole system devoted to this.
The main cells of the immune system are lymphocytes – large cells that destroy pathogens. There are two types of lymphocytes – B cells and T cells. T cells physically destroy pathogens, while B cells secrete antibodies specific for specific antigens. In fact, the classification includes many more cells than the two types, but this is a matter of advanced immunology. Most importantly, the body is able to train lymphocytes to "hunt" for the selected antigen. Once encountered with an antigen, the lymphocyte "remembers" it and the next time it reacts faster and more deadly.
But all these cells and molecular mechanisms come from various organs. In addition, a significant number of bacteria and other pathogens do not even reach the cells of the immune system, since they are eliminated by other organs and tissues.
Lymphoid tissue is not a separate tissue, and consists, for the most part, of connective and epithelial tissue. Lymphoid tissue is divided into central and peripheral, as well as encapsulated and non-encapsulated. The central lymphoid tissue includes the thymus and bone marrow. Peripheral lymphoid tissue includes accumulations of non-encapsulated lymphocytes in various organs, for example, in the intestines, respiratory tract, and other mucous membranes. Also, peripheral lymphoid tissue is lymph nodes and spleen.
Let's consider them in more detail.
Organs and their functions: skin and lymph nodes
The first barrier to pathogens is the mucous membranes (e.g., nasopharynx) and skin. This lymphatic tissue is non-encapsulated. These integuments have several factors that prevent the penetration of the pathogen. On the mucous membranes, there is not only mucus but also special ciliated cells that mechanically remove bacteria. In addition, mucus and saliva contain enzymes such as lysozyme (it kills most bacteria by literally dissolving them) and inhibin, which inhibits dangerous bacterial compounds. Although the skin does not secrete specific enzymes, it releases fatty acids that are bactericidal for some bacteria. The intestinal mucosa secretes hydrochloric acid, which is dangerous for most pathogenic bacteria. In addition, some digestive enzymes "work" against bacteria.
If the bacteria were able to overcome the skin and mucous barriers, then the lymph nodes perform a protective function. Lymph nodes are essentially filters within the lymphatic system, and they are encapsulated organs. Lymph nodes filter lymph and trap pathogens in their structures, which are then either destroyed by lymphocytes (subjected to phagocytosis) or provoke the development of inflammation, the most important protective reaction. Phagocytosis is carried out either by macrophage cells or microphage cells (they usually include neutrophils and basophils). Lymphocytes also work in the lymph nodes, "attracted" by inflammation.
Organs and their functions: bone marrow, spleen, and thymus
The most important organ for the formation of cells of the immune system is the bone marrow. Most cells are formed in the bone marrow: these are lymphocytes, monocytes (future macrophages), and granulocytes (which include neutrophils, basophils, and eosinophils). All these cells are necessary for the body's defense: for example, eosinophils are involved in the neutralization of worm eggs.
In the bone marrow, these cells are formed, but after entering the bloodstream, they need to mature. Freshly matured cells are called naive. Cell formation occurs after that, in other organs, by selection. T-lymphocyte selection takes place in the encapsulated thymus (which is why they are called T-cells). T-lymphocytes must be effective against pathogens, but they should not attack the body's own cells. Selection of B-lymphocytes that secrete antibodies occurs in the non-encapsulated lymphoid tissue of spleen and sometimes in the lymph nodes. Selection in a general sense requires that naive cells become "acquainted" with an antigen. This process of acquaintance is called antigen presentation. In addition, B-lymphocytes can also mature in the mucous membranes.
Conclusion
So, the body has several levels of protection against pathogens. Firstly, we are protected by mucous membranes and skin, which secrete substances that destroy bacteria and viruses. Then, the pathogen enters the lymph nodes, where there is a fight with the cells of the immune system. Immune tissue is divided into central and peripheral. The central one includes the bone marrow, where all cells are formed, and the thymus, where the maturation of T-lymphocytes takes place. In addition, there is peripheral tissue, which is located on the mucous membranes, in the spleen, and in the lymph nodes.