Dendritic cells: what are they, characteristics and functions in the body?

Dendritic cells are part of the immune system; let's see how they help us to stay healthy.

The immune system of living beings is the body's natural defense against viral, bacterial and parasitic infections, which also protects against intrinsic events in the organism, such as the formation of cancer cells and other deleterious processes. This set of elements allows us to maintain homeostasis (healthy internal balance), based on the recognition and destruction of what is considered harmful.

In general, if we think of the immune system, terms such as lymphocytes and other leukocytes come to mind, i.e. the white Blood cells circulating in the blood that signal, locate and eliminate agents that may cause us some kind of harm.

Although lymphocytes are essential for understanding the immune response, they are far from being the only cell bodies involved in the eradication of pathogens in our body. Today we are here to tell you all about dendritic cells, a very interesting cell type unique to mammals, essential for the innate immune response. essential for the innate (non-specific) and acquired immune response at the same time. Don't miss it.

What are dendritic cells?

According to the National Cancer Institute (NIH), a dendritic cell is a specialized type of cell body found in tissues such as the skin that stimulates immune responses by presenting an antigen on its surface.by presenting an antigen on its surface to other specialized cells.

Before proceeding further, we should clarify the term just introduced. An antigen can be defined as a substance which, when introduced into the body, induces an immune response in the organism, causing the formation of antibodies.

Antibodies or immunoglobulins are proteins circulating in the blood, whose function is to "signal" the antigen by adhering to it, so that other cellular bodies such as macrophages interpret it as a threat and eliminate it.

It may seem strange to bring up such specific terms, but, to put it mildly, the work of an antibody and an antigen is not the same, the work of an antibody and a dendritic cell are not very different.. Anyway, let's not get ahead of ourselves: it's time to explore some of the most striking features of this cell type.

Characteristics of dendritic cells

Mature dendritic cells (mDCs) have a morphology of their own, characterized by the presence of membranous extensions that may take the form of stars, pseudopods or veils. Due to their antigen-presenting nature, these cell bodies contain multiple structures specialized in this specific function, such as endosomes, lysosomes or Birbeck granules.

Endosomes are compartments within the cytoplasm that act as transporters of materials within animal and fungal cells. Lysosomes, on the other hand, act as destructive organelles, since they contain hydrolytic and proteolytic enzymes responsible for degrading cellular material, whether of external (heterophagy) or internal (autophagy) origin. Birbeck granules, present in the dendritic cells of the epidermis, have a function yet to be discovered.

These characteristic cell bodies are present in lymphoid (lymphatic system) and non-lymphoid tissues, such as the skin, pharynx, upper esophagus, vagina, anus and in the mucosal linings of the respiratory and intestinal tract. Their "tentaculate" shape makes sense, as the dendritic cells extend their protrusions between the tight cell junctions without altering the nature of the tissue.. As we will see below, this mechanism allows the cell to "span" more space for antigen capture.

Immature dendritic cells are the ones that present the typical stellate shape as they patrol the peripheral areas of the body. The dendrites themselves (cytoplasmic processes measuring more than 10 μm and protruding from the cell body) are in continuous movement in the cells, retracting and extending again, as if they were their own living being. Of course, the complex morphology of such a small element is impressive.

The function of dendritic cells

Dendritic cells were properly described in 1973 by R. M. Steinman, a Canadian immunologist, and his colleague ZA. Cohn. In Steinman's own words, "DCs are the natural sentinels of the immune system", as they decide whether or not to induce an adaptive immune response in the presence of potential pathogens. We explain this very complex process in the simplest way.

Dendritic cells patrol mammalian tissues, searching for and detecting potential pathogen antigens (viruses, bacteria and parasites) by means of their different surface receptors (PRRs). (viruses, bacteria and parasites) through their various surface receptors (PRRs). Once the potential threat is identified, the immature DC becomes a mature dendritic cell (mDC), releasing different proinflammatory substances involved in the innate response of the organism. In addition to this, they also have the capacity to capture and degrade antigens, thanks to a process called phagocytosis.

Once the dendritic cell "has" the antigen of the pathogenic agent in its domains, it migrates to the lymph nodes, where the lymphocytes are stored.where the T lymphocytes are stored. These lymphocytes "recognize" the antigen, and become activated, proliferate and attack the initial dangerous substance, this being part of the adaptive immune response.

It is very interesting to know that, of the differentiated lymphocytes, some will be effectors and cytokine secreters (inflammatory response), but others will remain as memory lymphocytes, that is, they will remember the antigen of the pathogen in order to respond more effectively to it on subsequent occasions.

Immune tolerance

Beyond all these essential functions, it is worth noting that dendritic cells protect us against ourselves. When T lymphocytes leave the thymus for the first time, dendritic cells control them.When the T lymphocytes first leave the thymus, the dendritic cells control them by destroying those carrying antigens that could cause the immune system to attack itself.

Imagine that, for example, the antigen is part of a red blood cell. If this lymphocyte were to proliferate and generate copies of itself, the effect on the organism could be disastrous.

Dendritic cells in figures

These cells are fascinating from both a histological and functional point of view, as they are able to present an antigen to lymphocytes they are able to present an antigen to lymphocytes without the lymphocytes having had any contact with the pathogen in any way.. They are considered to be the major activators of the adaptive immune system, as a single mDC is capable of stimulating, under experimental conditions, 100 to 3000 T lymphocytes.

In short: immature dendritic cells are the ones with cytoplasmic protrusions (dendrites), with which they "search" for antigens to present to the lymphocytes in almost all tissues of the organism. Once they detect one, they make it their own and quickly leave the affected tissue, travel through the lymphatic system, and "alert" the lymphocytes. They "warn" the lymphocytes agglutinated in the lymph nodes that they must act quickly and accurately.. Undoubtedly, this is one of the most fascinating immune mechanisms ever recorded in mammals.

Dendritic cells and immunotherapy

Not everything remains in a theoretical framework, as the excellence of dendritic cells holds real promise in the medical field. For example, it has been found that tumor cells suppress DCs by releasing certain compounds, causing them to stimulate the production of immune cells.The tumor cells, for example, have been found to suppress DCs by emitting certain compounds, causing them to stimulate the production of suppressor T lymphocytes, leukocytes that suppress the activation of the immune system. Thus, the tumor can grow freely without the body acting against it.

Surprising as it may seem, it has been possible to expose dendritic cells to tumor antigens under laboratory conditions, in order to inject them into the patient with neoplasia to reactivate his immune response. It has not been functional in all cases, but it is certainly a promising mechanism. Should a broad spectrum of efficacy be achieved in this methodology, we could be looking at the first vaccine against some types of cancer.. Fascinating, isn't it?

Summary

In this article we have presented very complex concepts, because explaining immune responses without getting into too specific terminology is, to say the least, a challenge. If we want to keep you with one idea, it is this: dendritic cells patrol our tissues and, when they observe a threat, they "hijack" their antigen and migrate to the site of the T lymphocytes, alerting them to act.

In addition to this, they also release various inflammatory substances of their own. DCs are therefore considered essential in both the innate and adaptive immune response. Of course, spaces like this make us realize what an engineering feat the human body really is.

Bibliographic references:

• Alfaro, C., Oñate, C., Rodríguez, A., Pérez-Gracia, J. L., Fernández de Sanmamed, M., & Melero, I. (2013, December). Dendritic cells specialized in exogenous antigen presentation to cytotoxic T lymphocytes. In Anales del Sistema Sanitario de Navarra (Vol. 36, No. 3, pp. 519-537).
• Dendritic Cells, British Society for Immunology. Retrieved January 9 from https://www.immunology.org/es/public-information/bitesized-immunology/cells/c%C3%A9lulas-dendr%C3%ADticas.
• Definition of dendritic cell, NIH. Retrieved January 9 from https://www.cancer.gov/espanol/publicaciones/diccionario/def/celula-dendritica.
• Functions and classification of dendritic cells, University of Cordoba. Retrieved January 9 from https://helvia.uco.es/xmlui/handle/10396/9467.
• Vázquez, M. B., Sureda, M., & Rebollo, J. (2012). Dendritic cells I: basic aspects of their biology and functions. Immunology, 31(1): pp. 21 - 30.