Antigen presenting cell
Antigen-presenting cells enable the detection of invading pathogens or altered body cells and thus initiate their effective elimination through a specific immune response . Although almost all body cells have the ability to present antigens , this term usually only refers to the so-called "professional" antigen-presenting cells of the immune system - that is, dendritic cells , monocytes , macrophages and B-lymphocytes . “Professional” here means that antigen presentation is an essential function of these cells and that only they can trigger activation of T lymphocytes that have not previously come into contact with the corresponding antigen . In this case, the antigen presentation takes place via so-called MHC II complexes . In contrast, most other body cells present antigens via MHC I in order to be recognized by the immune system even in the event of an infection or tumor-like change.
Dendritic cells
Dendritic cells have a large repertoire of molecules that can trigger and influence the activation of T lymphocytes . This gives them the ability to set in motion a fast and effective immune response against invading pathogens, especially if it is a first-time infection. Since they can also perceive a variety of inflammatory signals from the surrounding tissue and pass them on to T lymphocytes, they have a decisive influence on whether the developing immune reaction takes a humoral or cell-mediated course.
Dendritic cells are found in all body tissues. In the so-called immature state, they constantly absorb antigen in different forms: dead cells via phagocytosis , molecules contained in the body fluid via pinocytosis and specifically selected substances via receptor-mediated endocytosis . An infection and the associated inflammatory reaction trigger a maturation process in dendritic cells, in the course of which they lose the ability to take up new antigen. The already absorbed antigen is now presented effectively and in high density on the cell surface. During an inflammation, a variety of signals act on the dendritic cells, be they bacterial substances or inflammatory messenger substances released by the tissue. These stimulate dendritic cells to produce further factors, so that the presented antigen is integrated into a complex pattern of activating and inhibiting signals, which enables an exact adaptation of the immune response to the infection found. When maturation begins, dendritic cells migrate from the body tissues to the nearest lymphatic organ, where contact with T lymphocytes occurs. If a T lymphocyte and its T cell receptor recognize the presented antigen, there is a firm and long-lasting bond to the dendritic cell and a diverse signal exchange, which ensures that the immune response that follows the infection that has occurred is as efficient as possible can eliminate.
Monocytes and macrophages
Macrophages and their cellular precursors, the monocytes, eliminate damaged tissue and phagocytize pathogens that have penetrated. The antigen absorbed in the process is then presented directly in the same tissue, the cells usually do not migrate to lymphatic organs. Since the T lymphocytes found in the tissues have usually already had contact with antigen, macrophages are primarily involved in maintaining an immune reaction that has already started or in defending against infections with an already known pathogen. The microglial cells of the central nervous system represent a unique type of phagocytic cells, the specialty of which is that they are both glial cells of the nervous tissue and mononuclear phagocytes.
B lymphocytes
B-lymphocytes take up antigens via receptor-mediated endocytosis with the help of their B-cell receptor . If the antigen absorbed and subsequently presented is recognized by T lymphocytes, mutual activation of B and T lymphocytes occurs. B-lymphocytes then either release large amounts of antibodies that bind to the same antigen as the B-cell receptor, or they change their antibodies at random so that some cells can produce antibodies with increased effectiveness. The antigen presentation by B cells is therefore primarily used to control one's own antibody production.
Individual evidence
- ^ RM Ransohoff, AE Cardona: The myeloid cells of the central nervous system parenchyma . In: Nature . tape 468 , no. 7321 , 2010, p. 253-262 , PMID 21068834 .