In order for the immune system to function and protect the body from damage caused by diseases, all cells involved have a special task. They fit into the interaction of different immune cell types like little soldiers and make their contribution to the immune response. One of these cell types are dendritic cells.
dendritic cells are the main antigen-presenting cells of the immune system. In particular, they are responsible for the activation of T lymphocytes.
T lymphocytes are also important cells of the immune system. They are partly responsible for destroying pathogens and therefore need to be informed of their presence and activated. You can learn more about their function in the explanation of T cells/T lymphocytes or in the overview explanation of the immune system.
Dendritic cells – tasks and function
Dendritic cells are central in two ways immune system a: On the one hand, they can immune response initiate, on the other hand they protect the body by promoting tolerance autoimmune reactions.
Tolerance mediation ensures that the immune system shows tolerance to structures that come from its own body. The aim is to prevent the body from starting an immune response against itself and thereby triggering an autoimmune disease.
initiation of an immune response
Dendritic cells are basically found everywhere where there is contact with the outside world, i.e. in mucous membranes (nose, mouth, throat, esophagus, vagina, intestines, respiratory organs) and the Skin. There they are the first instance of immune cells that protects the body and are therefore also referred to as the «guardians» of the immune system.
Foreign body detection
if it pathogens how bacteria or viruses create through the skin barrier or mucous membranes, dendritic cells are there to intercept them. They use their movable arms to look out for pathogens and phagocytosis initiate as soon as they come into contact with them.
phagocytosis describes a process in which solids pass through the membrane of a cell into vesicles be included. The contents of this vesicle are then broken down by special substances and thus rendered harmless.
Once a pathogen or components of the pathogen have been recognized and ingested, they can be cut up. This resulting exciter pieces – also as antigens designated – are then used to alert other components of the immune system that an intruder has been sighted.
Alerting the immune system
To tell other immune cells which invader to look out for, the antigens are placed in special molecules found on the surface of the dendritic cells. These are also called MHC-I or MHC-II molecules designated.
MHC-II molecules present antigens that have been taken up and degraded by phagocytosis. MHC-I molecules, on the other hand, are occupied by antigens that were previously in the cell. This is the case, for example, with an infection viruses the case. viruses enter the dendritic cell to infect it, but unknowingly make their components available to be presented in MHC-I molecules. So:
- MHC-I molecules present intracellular fragments
- MHC-II molecules present extracellular fragments
In addition, the dendritic cells must move to where other immune cells are located in order to present them with evidence of infection. For this they migrate secondary lymphatic organs out, about the lymph nodes. There they meet naive T cells (CD4+ or CD8+ cells) and can activate them. To do this, the receptor on a T cell must interact with the antigen in the MHC molecule.
receptors are often found on the outside of cells, waiting like a lock to bind a molecule that fits them like a key. In this case, the molecule is an antigen sitting on an MHC molecule. An action is then triggered in the cell in response to the binding.
The two MHC molecules are responsible for activating different ones T cells responsible:
- MHC-I: activates CD8+ T cells into cytotoxic T cells
- MHC-II: activates CD4+ T cells into T helper cells
The peculiarity, naive T cells being able to activate allows the dendritic cells to trigger a primary immune response (= the immune reaction of the body after initial contact with a pathogen). the immune response itself is mainly carried out by other components of the immune system. Nevertheless, dendritic cells, e.g. B. through the release of cytokines involved.
Simply presenting an antigen in the MHC molecule and allowing the T cell receptor to bind it is not enough to activate a T cell. There are also other signals that have to be exchanged between the T cell and the dendritic cell in order to achieve full activation. These are also called co-stimulatory signals designated.
tolerance mediation
Just as important as the efficient initiation of a defensive reaction is protection against autoimmune diseases. In the MHC molecules, fragments of inner– and presented outside the cell, which is why it may also include pieces of the body’s own cells. Since no distinction is made between components of pathogens and body cells when antigens are presented, this step must be taken over by immune cells that can recognize antigens in MHC molecules.
A common source of fragments from endogenous cells is the apoptosis.
In apoptosis, cells initiate their own death once they reach a certain age or realize they could become a threat through mutation/infection. This protective mechanism causes the cells to die and slowly break down into their components. This creates fragments that are found by immune cells and loaded into their MHC-II molecules.
To prevent that immune system to endogenous antigens – the so-called self antigens – reacts, dendritic cells are used.
Self antigens can be collected by dendritic cells and presented to T cells in secondary lymphoid organs. However, they should T cells be stimulated there to an immune reaction. Instead, the dendritic cells contribute to making all self-reactive T cells be recognized and eliminated. The corresponding T cells go z. B. apoptosis a or differentiate to regulatory T cells, another subset of T cells.
In this way it is ensured that no T-cells react against structures of the body’s own cells and thereby trigger diseases such as e.g. B. at Multiple sclerosis (MS) the case is.
If you want to know more about the disease multiple sclerosis, then have a look at the corresponding explanation.
Dendritic cells – structure and maturation
The name «dendritic cells» derives from the Greek word «dendrites» from, which means something like «tree-like» or «branched». Accordingly, you can also imagine their characteristic structure.
Immature dendritic cells have a cell body with long, star-shaped processes radiating out in all directions (dendrites). These are mobile and can therefore scan their surroundings.
Danger! You should not confuse the cell processes of the dendritic cells with the dendrites of the nerve cells.
Dendritic cells arise from stem cells in the bone marrow. However, many other immune cells can also develop from these stem cells, and research is still being carried out into which factors are precisely responsible for the development of dendritic cells.
Maturation of dendritic cells
Immature dendritic cells have MHC molecules on their surface, the function of which you have already been able to get to know. After contact with antigens, dendritic cells migrate in secondary lymphatic organs. These are, for example, the lymph nodes or the spleen. Along the way, the dendritic cells mature, changing both their shape and the number of receptors on their surface.
Instead of the many dendrites, the membrane changes into protuberances that form mysterious structures. MHC molecules multiply on their surface and B7 molecules are formed, who have not yet been present. In addition, the dendritic cells lose the ability to carry out phagocytosis. All of these changes support the functions that dendritic cells now have to fulfill as part of the immune response.
Dendritic cells – types and subclasses
Dendritic cells are not a single mass of cells, but rather different types of immune cells that share similar structures and functions. They can be classified based on different characteristics and divided into subclasses.
Classification according to surface characteristics
Immune cells have different types on their surfaces CD molecules (CD = clusters of differentiation). They enable the immune cells to perform various functions. However, CD molecules can also be used to characterize immune cells, since each type of immune cell has a specific combination of CD molecules.
When classifying dendritic cells based on the CD molecules on their surface, two types are distinguished.
Myeloid dendritic cells
The myeloid dendritic cells are a widespread, quite heterogeneous group of cells responsible for the uptake, processing and presentation of antigens. Because they represent the largest group of dendritic cells, they are also referred to as conventional dendritic cells. They resemble those monocytes and possess the surface markers CD11c and CD33.
Plasmacytoid dendritic cells
Plasmacytoid dendritic cells are rare compared to myeloid ones and once activated resemble one plasma cell.
Plasma cells are mature B cells that can produce appropriate antibodies after contact with an antigen. Have plasma cells aroused your interest? Then take a look at the explanation of B cells!
Plasmacytoid dendritic cells have the surface markers CD123, CD303 and CD304, but not CD11c, which is characteristic of myeloid dendritic cells. In addition, they are hMainly relevant for defense against viruses. They can synthesize large amounts of antiviral substances (cytokines) such as type I interferons. In infections such as HIV, the number of plasmacytoid dendritic cells is reduced.
Further classifications of dendritic cells
In addition to surface features, dendritic cells can also be classified based on other features. For this purpose, specific functions and localizations can be taken into account.
Langerhans cells
Langerhans cells are specialized dendritic cells of the skin and mucous membranes. e.g. For example, pathogens that enter the body after injuries to the skin are phagocytosed by Langerhans cells. They, too, then migrate to the lymph nodes…