Branches of immunity
The immune system
Alexander Apt, Post-science
What is the immune system? This is a very ancient evolutionary thing that originated in living nature before the division of plants and animals into kingdoms, because plants also have their own immune system, and, of course, animals also have it. The main function of this system is protection from invasion from the outside, that is, protection from infections, infestations, parasites, microbes. You can also add a reaction to allergens to this – this is also one of the parts of immunity, but the primary tasks of the immune system are to fight someone else's.
To do this, there are many branches in the immune system, and, in particular, the entire immunity can be divided into innate and adaptive, or acquired. They differ by a very simple criterion: innate immunity does not require any training of the immune system, it immediately reacts when it sees someone else's (and how it sees is a separate story), but there is no concept of memory in innate immunity, since it will respond to any new invasion in the same way as the previous one. Adaptive immunity is a newer thing in evolution, it is therefore adaptive because it adapts to the invasion of the parasite, the host begins to react. The first time he reacts slowly and weakly, and with repeated contacts he is already trained, his receptors are tuned to this type of invasion, and the host reacts stronger and more effectively. These are two branches of immunity.
The immune response develops in special immune organs – in the lymph nodes and spleen, but in order for these organs to be stuffed with cells of the immune system, they must go there. And they come from one source – bone marrow. In the bone marrow of animals, all types of immune system cells are born from precursors, some already in a ready-to-function form, for example, cells such as neutrophils that enter the blood from the bone marrow, monocytes that enter the blood from the bone marrow and then in organs can turn into macrophages, another type of phagocytosis cells. And there are, of course, lymphocytes that are mainly engaged (almost all types of lymphocytes, but not all) in the adaptive phase, and they are being trained. This is especially true of cells such as T-lymphocytes, which, after leaving the bone marrow, enter another central organ of the immune system – the thymus.
The thymus is such a thymus gland, the formation that lies on the heart is located right in the mediastinum, and there is absolutely wonderful in its subtlety and accuracy work on the selection of T–lymphocytes for subsequent functioning as cells of the immune system. All lymphocytes that overreact to their own are destroyed there, because cells that can react to both their own and someone else's come out of the bone marrow. But in the thymus there is a negative, negative selection of those T-cells that can react too strongly to their own, and those T-cells that do not recognize someone else's. And 95% of the cells entering there die in the thymus. The remaining 5% go to the periphery, to the lymphatic system, populate the spleen and lymph nodes and are there ready to receive the incoming signal from cells that are in primary contact, for example, with a microbe, a virus, and then deliver this material to lymphocytes, which begin either to produce antibodies, if these are B-lymphocytes, or directly contact with phagocytes and produce a variety of mediators, immunologically active molecules that help the host fight the parasite. And to one degree or another, such a system appears in evolution approximately from fish and, constantly improving, reaches higher mammals, in particular humans. In lower animals and plants, immunity is arranged differently, and there innate immunity is much more important than adaptive. In our country, they are approximately equal.
Of course, it is impossible not to mention such a function of the immune system as the fight against tumors. It is unlikely that this was born evolutionarily, as a directly primary necessary function. Most likely, evolution generally picks up everything that is useful. Most likely, the system that was developed to fight with someone else turned out to be to some extent suitable for fighting with one's own, which was reborn. This is the fight against tumors. In this aspect, there are cells of innate immunity, the so–called natural killers (or natural killers) are cells that can react with tumor cells and directly destroy it. And there are cells of the adaptive system, part of the T-lymphocytes, and this is a large and important part, it is able to destroy tumors, acquiring immunity to tumor antigens. True, this is not as effective as immunity against microbes, but nevertheless, it seems to save us in very many cases without the intervention of medicine. Just such a built-in mechanism.
Immunity works with different types of pathogens in different ways. If the pathogens are extracellular – and these, for example, are banal for all of us streptococci, staphylococci, intestinal infections – then there are primarily innate immunity cells that simply phagocytize and destroy the microbe, and B-lymphocytes that produce antibodies. These are cells of the adaptive immune system, and their main functions are antibodies to pathogens. And there is, of course, a very large group of intracellular pathogens, primarily all viruses. Secondly, these are quite a few types of bacteria and protozoa. In particular, they include mycobacterium tuberculosis (the subject of my specialization), on which antibodies do not act at all, because they are inside the host cells for most of the life cycle, so they simply cannot get there from the outside. Or even an attempt to neutralize such pathogens, methods that are suitable for extracellular microbes, lead to unpleasant consequences.
In particular, an excessive influx of neutrophils into the focus of intracellular infection can lead to the appearance of purulent inflammation, but it absolutely cannot cope with a microbe that sits, for example, inside macrophages, even larger than neutrophils, which cannot absorb them simply by definition. But primary phagocytosis is very, very effective against rapid acute conditions, where it is absolutely necessary. The main active principles of immunity are phagocytes, lymphocytes, their products – antibodies, cytokines that activate other cells, the immune system includes auxiliary molecules. For example, there is such a compliment system that helps the toxic effect of antibodies against pathogens.
How else do the two immune systems differ? They differ greatly in the receptors that recognize the intrusion. Innate immunity cells have several families of receptors on the surface that directly see the pathogen. And the cells of adaptive immunity have a very complex system of receptors, which, with the development of infection, undergo complex structural rearrangements and become specific to this type of infection.
Immunology is developing very quickly, and, in addition to such inevitable aspects in science as the description of new molecules, an attempt to classify everything and everything that science has always sinned, there are real breakthroughs. In particular, we have only now begun gradually, after the work of the 1990s and 2000s, to understand how the connection between the ancient innate immunity and a much more evolutionarily new adaptive one works. This is probably one of the hot spots of immunology right now: how signals from one recognition system go to another recognition system and how orchestrated it all is, in concert it works against diseases.
About the author:
Alexander Apt – Doctor of Biological Sciences, Professor of the Department of Immunology of the Faculty of Biology of Moscow State University, Head of the Laboratory of Immunogenetics of TSNIIT RAMS
Portal "Eternal youth" http://vechnayamolodost.ru
16.06.2016