What is antigen presentation and why is it important?
What is antigen presentation and why is it important? Antigen presentation serves to ensure adaptive immune responses are initiated to invading microorganisms. Therefore, in an effort to survive in the host, pathogens target antigen presentation pathways and disable their function.
Why is antigen presentation important in fighting infection?
Antigen-presentation recognizes the virus so that the cells can fight the infection. Why is antigen presentation important in fighting infection? They can only recognize one type of antigen at once because they are specified cells that always bind to the same antigen.
What does antigen presentation mean?
Which ones are antigen presenting cells quizlet?
- Dendritic cell – Essential for naive T-cell presentation and triggering a primary response.
- B-cell – Present antigen on MHCII to memory T-helper cells.
- Macrophage – Present antigen on MHCII to memory T-helper cells.
What is the best antigen?
The clinical performance of rapid antigen diagnostic tests largely depends on the circumstances in which they are used. Rapid antigen tests perform best when the person is tested in the early stages of infection with SARS-CoV-2 when viral load is generally highest.
What are MHC class II molecules?
MHC class II molecules are expressed by APCs, such as dendritic cells (DC), macrophages and B cells (and, under IFNγ stimuli, by mesenchymal stromal cells, fibroblasts and endothelial cells, as well as by epithelial cells and enteric glial cells). MHC class II molecules bind to peptides that are derived from proteins degraded in the endocytic pathway. MHC class II complexes consists of α- and β-chains that are assembled in the ER and are stabilised by invariant chain (Ii). The complex of MHC class II and Ii is transported through the Golgi into a compartment which is termed the MHC class II compartment (MIIC). Due to acidic pH, proteases cathepsin S and cathepsin L are activated and digest Ii, leaving a residual class II-associated Ii peptide (CLIP) in the peptide-binding groove of the MHC class II. Later, the CLIP is exchanged for an antigenic peptide derived from a protein degraded in the endosomal pathway. This process requires the chaperone HLA-DM, and, in the case of B cells, the HLA-DO molecule. MHC class II molecules loaded with foreign peptide are then transported to the cell membrane to present their cargo to CD4+ T cells. Thereafter, the process of antigen presentation by means of MHC class II molecules basically follows the same pattern as for MHC class I presentation.
What is the difference between MHC class I and class II?
The difference is that the peptides originate from different sources – endogenous, or intracellular, for MHC class I; and exogenous, or extracellular for MHC class II. There is also so called cross-presentation in which exogenous antigens can be presented by MHC class I molecules. Endogenous antigens can also be presented by MHC class II ...
What are the genes that encode MHC class I?
Human MHC class I molecules are encoded by a series of genes – HLA-A, HLA-B and HLA-C (HLA stands for ‘Human Leukocyte Antigen’ , which is the human equivalent of MHC molecules found in most vertebrates). These genes are highly polymorphic, which means that each individual has his/her own HLA allele set. The consequences of these polymorphisms are differential susceptibilities to infection and autoimmune diseases that may result from the high diversity of peptides that can bind to MHC class I in different individuals. Also, MHC class I polymorphisms make it virtually impossible to have a perfect tissue match between donor and recipient, and thus are responsible for graft rejection.
What is the process of presenting endosomal peptides?
Some of the MHC class I molecules can be recycled and present endosomal peptides as a part of a process which is called cross-presentation. The usual process of antigen presentation through the MHC I molecule is based on an interaction between the T-cell receptor and a peptide bound to the MHC class I molecule.
What are the three genes that make up the MHC class II?
MHC class II polymorphism. Like the MHC class I heavy chain, human MHC class II molecules are encoded by three polymorphic genes: HLA-DR, HLA-DQ and HLA-DP. Different MHC class II alleles can be used as genetic markers for several autoimmune diseases, possibly owing to the peptides that they present.
What is the function of MHC class I?
On the surface of a single cell, MHC class I molecules provide a readout of the expression level of up to 10,000 proteins. This array is interpreted by cytotoxic T lymphocytes and Natural Killer cells, allowing them to monitor the events inside the cell and detect infection and tumorigenesis.
What amino acids are translocated in a tap?
TAP translocates peptides of 8 –16 amino acids and they may require additional trimming in the ER before binding to MHC class I molecules. This is possibly due to the presence of ER aminopeptidase (ERAAP) associated with antigen processing.
What is the presentation of native intact antigens to B cells?
Presentation of native intact antigens to B cells. B-cell receptors on the surface of B cells bind to intact native and undigested antigens of a structural nature , rather than to a linear sequence of a peptide which has been digested into small fragments and presented by MHC molecules. Large complexes of intact antigen are presented in lymph nodes ...
Why do T cells recognize only fragmented antigens displayed on cell surfaces?
Because T cells recognize only fragmented antigens displayed on cell surfaces, antigen processing must occur before the antigen fragment, now bound to the major histocompatibility complex (MHC), is transported to the surface of the cell, a process known as presentation , where it can be recognized by a T-cell receptor.
What are cytotoxic T cells?
Cytotoxic T cells (also known as T c, killer T cell, or cytotoxic T-lymphocyte (CTL)) express CD8 co-receptors and are a population of T cells that are specialized for inducing programmed cell death of other cells. Cytotoxic T cells regularly patrol all body cells to maintain the organismal homeostasis. Whenever they encounter signs of disease, caused for example by the presence of viruses or intracellular bacteria or a transformed tumor cell, they initiate processes to destroy the potentially harmful cell. All nucleated cells in the body (along with platelets) display class I major histocompatibility complex (MHC-I molecules). Antigens generated endogenously within these cells are bound to MHC-I molecules and presented on the cell surface. This antigen presentation pathway enables the immune system to detect transformed or infected cells displaying peptides from modified-self (mutated) or foreign proteins.
What type of MHC binds peptides?
There are two types of MHC molecules which differ in the behaviour of the antigens: MHC class I molecules (MHC-I) bind peptides from the cell cytosol, while peptides generated in the endocytic vesicles after internalisation are bound to MHC class II (MHC-II).
What are the main proteases in endosomes?
The main proteases in endosomes are cathepsins and the result is the degradation of the antigens into oligopeptides. MHC-II molecules are transported from the ER to the MHC class II loading compartment together with the protein invariant chain (Ii, CD74).
What is the process of internalising antigens in APCs?
APCs usually internalise exogenous antigens by endocytosis, but also by pinocytosis, macroautophagy, endosomal microautophagy or chaperone-mediated autophagy. In the first case, after internalisation, the antigens are enclosed in vesicles called endosomes.
Which enzymes are involved in the antigen presentation pathway?
There are three compartments involved in this antigen presentation pathway: early endosomes, late endosomes or endolysosomes and lysosomes, where antigens are hydrolized by lysosome-associated enzymes (acid-dependent hydrolases, glycosidases, proteases, lipases). This process is favored by gradual reduction of the pH.
What class of antigens are present in cross-presentation?
Whilst this is the general rule, in cross-presentation extracellular antigens can be presented by MHC class I and in autophagy intracellular antigens can be presented by MHC class II. Before an antigen can be presented, it must first be processed. Processing transforms proteins into antigenic peptides.
What is the route of processing for exogenous antigens for MHC class II presentation?
The route of processing for exogenous antigens for MHC class II presentation begins with endocytosis of the antigen. Once inside the cell, they are encased within endosomes that acidify and activate proteases, to degrade the antigen.
How are antigens delivered to the surface of APCs?
Antigens are delivered to the surface of APCs by Major Histocompatibility Complex (MHC) molecules. Different MHC molecules can bind different peptides. The MHC is highly polygenic and polymorphic which equips us to recognise a vast array of different antigens we might encounter.
How do T cells recognize antigens?
T cells can only recognise antigens when they are displayed on cell surfaces. This is carried out by Antigen-presenting cells (APCs), the most important of which are dendritic cells, B cells and macrophages. APCs can digest proteins they encounter and display peptide fragments from them on their surfaces for another immune cell to recognise. This process of antigen presentation allows T cells to “see” what proteins are present in the body and to form an adaptive immune response against them. In this article we shall discuss antigen processing, presentation and recognition by T cells.
What is the co-receptor of TCR?
Co-Receptors. As well as the TCR, another T cell molecule is required for antigen recognition and is known as a co-receptor. These are either a CD4 or CD8 molecule: CD4 is present on T helper cells and only binds to antigen-MHC II complexes. CD8 is present on cytotoxic T cells and only binds to antigen-MHC I complexes.
What does cytotoxic T cells do?
Cytotoxic T cells will kill the cells that they recognise, whereas T helper cells have a broader range of effects on the presenting cell such as activation to produce antibodies (in the case of B cells) or activation of macrophages to kill their intracellular pathogens.
Which cells are most important for antigen presentation?
This is carried out by Antigen-presenting cells (APCs), the most important of which are dendritic cells, B cells and macrophages. APCs can digest proteins they encounter and display peptide fragments from them on their surfaces for another immune cell to recognise. This process of antigen presentation allows T cells to “see” what proteins are ...
How does antigen work?
The antigen acts as an antibody generator and it gets eliminated (along with the infectious agent) by the body's immune system. Antigens may not be the main attraction when it comes to immunity, but they play a crucial role in the prevention and elimination of diseases.
How are antibodies created?
Antibodies are created by cells within the immune system. They bind to antigens and promote the elimination of threatening pathogens from the body. They neutralize the threat by alerting other parts of the immune system to take over.
What is the significance of antigens?
Significance. Testing Relevance. An antigen is a molecule that stimulates an immune response by activating leukocytes (white blood cells) that fight disease. Antigens may be present on invaders, such as bacteria, viruses, parasites, fungi, and transplanted organs, or on abnormal cells, such as cancer cells.
How does the immune system respond to a viral infection?
In Viral Infection. In a viral infection such as the seasonal flu, the immune system develops a response by creating antibodies that can bind to the specific antigen. The process works in a similar way as it would with a vaccine, although the infectious viral germs are much stronger.
What is the importance of blood and tissue antigen testing?
Blood and Tissue Antigen Testing. Testing for different blood or tissue antigens is a very important aspect of blood transfusion or tissue or organ transplant. In the case of blood transfusion, blood types must match A, B, and O antigens between donor and recipient.
Why is it important to recognize antigens?
1 When the body identifies an antigen, it will initiate an immune response. When receptors on white blood cells bind to antigens, this triggers white blood cell multiplication and starts the immune response.
Which cells are responsible for binding antibodies?
They can be bound by white blood cells, including leukocytes, which are the cells of the adaptive immune system. Leukocytes include B cells and T cells. B cells make antibodies that can also bind to antigens. After an antigen gets bound to a B cell receptor, antibodies are produced.
Why is MHC so difficult to evade?
It contains several different MHC-I and MHC-II genes so that every individual possesses a set of MHC molecules with different ranges of peptide-binding specificities.
What is the difference between MHC-I and MHC-II?
The main consequence of this difference is that the ends of a peptide bound to an MHC-I molecule are buried within the molecule whereas the ends of peptides bound to MHC-II molecules are not. This difference allows more flexibility in the length and types of peptides that MHC-II molecules can bind.
What are antigens derived from?
Antigens are often categorized according to whether they are derived from (1) viruses, intracellular bacteria, or protozoan parasites (endogenous pathogens); or (2) exogenous pathogens that replicate outside of the cell.
What are the variants of each gene?
There are multiple variants of each gene within the population as a whole. The different variants that are inherited by an individual from a parent are known as alleles. The numbers of alleles recognized at the classical loci are presented in Table 10-1.
What is the MHC III region?
The MHC-III region includes several genes involved in the complement cascade (C4A, C4B, C2, and FB) (see section 6, Complement), the TNF-a and TNF-b (LTa) genes, the CYP21 gene that encodes an enzyme in steroid metabolism, the HSP70 gene that encodes a chaperone, and many other genes of unknown immunological function.
How are endogenous proteins synthesized?
After trimming by cytosolic proteases (3), the peptides enter the endoplasmic reticulum via the TAP 1 and TAP 2 transporters (4).
When was the MHC gene first recognized?
For that reason, the gene complex was termed the ‘‘major histocompatibility complex.’’. MHC genes (called the H-2 complex in mice) were first recognized in 1937 as a barrier to transplantation in mice.
Overview
Antigen presentation is a vital immune process that is essential for T cell immune response triggering. Because T cells recognize only fragmented antigens displayed on cell surfaces, antigen processing must occur before the antigen fragment, now bound to the major histocompatibility complex (MHC), is transported to the surface of the cell, a process known as presentation, where …
Presentation of intracellular antigens: Class I
Cytotoxic T cells (also known as Tc, killer T cell, or cytotoxic T-lymphocyte (CTL)) express CD8 co-receptors and are a population of T cells that are specialized for inducing programmed cell death of other cells. Cytotoxic T cells regularly patrol all body cells to maintain the organismal homeostasis. Whenever they encounter signs of disease, caused for example by the presenc…
Presentation of extracellular antigens: Class II
Antigens from the extracellular space and sometimes also endogenous ones, are enclosed into endocytic vesicles and presented on the cell surface by MHC-II molecules to the helper T cells expressing CD4 molecule. Only APCs such as dendritic cells, B cells or macrophages express MHC-II molecules on their surface in substantial quantity, so expression of MHC-II molecules is more cell-specifi…
Presentation of native intact antigens to B cells
B-cell receptors on the surface of B cells bind to intact native and undigested antigens of a structural nature, rather than to a linear sequence of a peptide which has been digested into small fragments and presented by MHC molecules. Large complexes of intact antigen are presented in lymph nodes to B cells by follicular dendritic cells in the form of immune complexes. Some APCs expressing comparatively lower levels of lysosomal enzymes are thus less likely to digest the an…
See also
• Immune system
• Immunology
• Immunological synapse
• Trogocytosis
External links
• ImmPort - Gene summaries, ontologies, pathways, protein/protein interactions and more for genes involved in antigen processing and presentation
• antigen+presentation at the US National Library of Medicine Medical Subject Headings (MeSH)