The most important step 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 that has entered the body and begin to perform their functions. To activate the T-lymphocyte, special helpers are needed - antigen-presenting cells (APCs), which present a fragment of foreign material on their surface as part of a major histocompatibility complex of the second class (MHC II).
MHC II are special molecules to which the surface T-helper receptors are specific.
The concept of antigen-presenting cells
APCs are the helper cells of the immune system. Among them there are "professionals" who can "turn on" native T-helpers, not only presenting an antigen, but also producing an inducing signal upon contact. Activated T-lymphocytes acquirethe ability to recognize foreign fragments on the membrane surfaces not only of APC, 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-helper cells, which have never been in contact with foreign agents, can only interact 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 molecules of the main histocompatibility complex of the second class on the surface.
The APC population is a heterogeneous group of leukocytes with pronounced immunostimulating properties. It includes several types of cells that are able to absorb foreign agents by phago- or endocytosis and expose them to the surface as part of receptors that can be recognized by T-helpers upon contact. The latter trigger a whole cascade of immune responses, which emphasizes the importance of APC.
Functioning of the AIC
The function of antigen-presenting cells is not only to present, but also to induce a specific signal that, upon contact, activates a native T cell that has never encountered an antigen.
The work of the AIC consists of two stages:
- processing - restriction of an antigen molecule into small fragments;
- presentation - embedding the antigenic peptide into MHC and exporting the resultingcomplex on the membrane surface.
Most of the APC is formed in the bone marrow.
When an antigen-presenting cell contacts 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 is carried out.
Which cells are considered antigen-presenting
In immunology, antigen-presenting cells are cells that are capable of:
- express second-class MHC molecules on the membrane surface;
- induce a stimulatory signal to the T cell population.
An especially important criterion is the presentation of the antigen in combination with MHC II, which can be recognized by T-helper. Almost all cells are capable of processing a foreign molecule as part of MHC 1, but they are not called antigen presenters.
Varieties of APK
In immunology, antigen-presenting cells are divided into two large groups: professional and non-professional.
Professional AICs include:
- macrophages;
- dendritic cells;
- B-cells.
The population of dendritic cells is quite extensive and subdivided into:
- white outgrowth epidermocytes (Langerhans cells);
- interdigital thymic cells;
- follicular dendritic cells (FDC).
All specialized APCs have the ability to deliver costimulatory signals to native T-lymphocytes, which is calledsensitization function.
Unprofessional APKs are:
- brain glial cells;
- epithelial cells of thymus and thyroid gland;
- endothelial vascular cells;
- pancreas beta cells;
- dermal 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 APC in the body
Antigen-presenting cells are predominantly located 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 combination with MHC, they migrate to the regional lymph nodes, where they interact with T-lymphocytes. The movement of the APC of Langerhans is carried out along the afferent lymphatic vessels.
A specific population of follicular dendritic cells (FDCs) responsible for antigen presentation to B-lymphocytes is concentrated in the lymphoid tissue of the mucous membranes and in the follicles of the lymph nodes.
The peculiarity of FDCs is that they do not migrate in response to infection, but are constantly part of a stable network formed by their own processes, which are connected to each other through desmosomes.
Angen presentation mechanism
As alreadyAs noted above, antigen presentation precedes processing. Initially, the antigen-presenting cell engulfs the 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 of 8-12 amino acid residues long.
Exogenous peptides that enter the APC are products of phagocyte digestion. In the antigen-presenting cell, they are further restricted to smaller peptides. Endogenous peptides are processed in proteasomes.
Then, the antigen fragment joins with the major 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.
