The most important stage in the development of a specific cellular immune response is the activation of the T-lymphocyte population. However, these cells cannot independently recognize a foreign agent entering the body and begin to perform their functions. To activate the T-lymphocyte, special assistants are needed - antigen-presenting cells (APCs), which represent a fragment of foreign material on its surface as part of the main histocompatibility complex of the second class (MHC II).
MHC II are specific molecules to which surface T-helper receptors are specific.
The concept of antigen presenting cells
APCs belong to auxiliary cells of the immune system. Among them there are "professionals" who know how to "incorporate" native T-helpers into their work, not only presenting the antigen, but also producing an inducing signal upon contact. Activated T-lymphocytes acquire the ability to recognize foreign fragments on the membrane surfaces of not only APCs, but also of all other cells capable of presentation. However, in the latter case, the antigen appears as part of MHC I, not II.
Native T-helpers, who have never been in contact with foreign agents, can interact only with the antigen-MHC II complex, which is formed exclusively in the APC. Thus, antigen-presenting cells of the immune system are cells capable of expressing the second class of histocompatibility complex molecules of the main complex.
AIC population is a heterogeneous group of white blood cells with pronounced immunostimulating properties. It includes several types of cells that are able to absorb foreign agents by phage or endocytosis and expose them on the surface as part of receptors that can be recognized by T-helpers upon contact. The latter, however, launch a cascade of immune response reactions, which emphasizes the importance of APC.
AIC functioning
The function of antigen-presenting cells consists not only in presentation, but also in the induction of a specific signal, which, upon contact, activates a native T-cell that has never encountered an antigen.
The work of the agro-industrial complex consists of two stages:
- processing - restriction of an antigen molecule into small fragments;
- presentation - embedding an antigenic peptide in the MHC and exporting the resulting complex to the membrane surface.
Most of the APC is formed in the bone marrow.
Upon contact of the antigen-presenting cell with a T-lymphocyte, the receptors of the latter recognize the MHC molecule modified by the incorporation of a foreign peptide. In this case, the effect of costimulation.
What cells are considered antigen-presenting
In immunology, antigen-presenting cells are cells that are capable of:
- Express on the membrane surface of the second-class MHC molecule;
- induce a stimulating signal for a population of T-lymphocytes.
A particularly important criterion is the presentation of the antigen precisely in combination with MHC II, which can be recognized by the T helper. Almost all cells are capable of processing a foreign molecule in MHC 1, but they are not called antigen-presenting.
Varieties of APK
In immunology, antigen-presenting cells are divided into two large groups: professional and non-professional.
Professional agribusiness include:
- macrophages;
- dendritic cells;
- B cells.
The population of dendritic cells is very extensive and is divided into:
- white process epidermocytes (Langerhans cells);
- thymus interdigital cells;
- follicular dendritic cells (FDK).
All specialized APCs have the ability to deliver co-stimulatory signals to native T-lymphocytes, which is called the sensitization function.
Unprofessional agribusiness are:
- brain glia cells;
- thymus and thyroid epithelial cells;
- endothelial vascular cells;
- pancreatic beta cells;
- skin fibroblasts.
Non-specialized APCs are able to form and excrete antigen-MHC II complexes only after stimulation with cytokines, which can be interferon-gamma and other substances.
Localization and migration of agro-industrial complex in the body
Antigen-presenting cells are located mainly in:
- skin
- lymph nodes;
- thymus;
- epithelium and subepithelial layer of most mucous membranes.
APCs concentrated in the epidermis are called Langerhans cells. After presenting the antigen on the surface in complex with MHC, they migrate to the regional lymph nodes, where they interact with T-lymphocytes. The movement of the agro-industrial complex Langerhans is carried out by afferent lymphatic vessels.
A special population of follicular dendritic cells (PDK), responsible for the presentation of antigens to B-lymphocytes, is concentrated in the lymphoid tissue of the mucous membranes and in the follicles of the lymph nodes.
A feature of FDK is that they do not migrate in response to infection, but are constantly part of a stable network formed by their own processes that connect to each other through desmosomes.
Antigen presentation mechanism
As noted above, antigen presentation is preceded by processing. Initially, an antigen-presenting cell absorbs a foreign agent by phagocytosis or endocytosis. Then, in special organelles (phagosomes or proteosomes) with the help of enzymes, antigenic proteins are cut into small fragments with a length of 8-12 amino acid residues.
Exogenous peptides that enter the APC are products of the digestion of phagocytes. In the antigen-presenting cell, they are further restricted to smaller peptides. Endogenous peptides are processed in proteosomes.
Then the antigen fragment is connected to the main histocompatibility complex. In the spatial conformation of the MHC molecule, there is a special cavity where the foreign peptide is placed. The resulting antigen-MHC complex is transported to the surface of the APC membrane.