Helper T Cells Function In The Adaptive Immune System Activation – MHC molecules also play an important role in directing the adaptive immune system. There are two main classes of MHC molecules: MHC class I and MHC class II.

MHC I glycoproteins are present on all nucleated cells in the body (they are not present on red blood cells or platelets). The function of MHC class I molecules is to take pieces of any protein synthesized within the cell

Helper T Cells Function In The Adaptive Immune System Activation

Helper T Cells Function In The Adaptive Immune System Activation

And “present” them on the cell surface. Cells constantly renew cellular proteins, removing old ones and replacing them with new ones. As part of this process, recycled proteins are broken down into small fragments called peptides and these are sent to the endoplasmic reticulum where some peptide fragments bind to a groove on the surface of the newly synthesized MHC class I molecules. The MHC-peptide complex is then transported to the cell surface and inserted into the cell membrane so that the peptide fragment is “presented” outside the cell where it is accessible to lymphocytes.

Adaptive Immune System

This mechanism becomes extremely valuable if a cell becomes infected with a virus or undergoes a malignant transformation (becomes cancerous). Viruses are unable to reproduce on their own; they must use the host cell’s synthetic “machinery” to make copies of viral components, including viral proteins. Some of these viral proteins will also be broken down into peptide fragments and combined with MHC class I molecules on the cell surface.

If a cytotoxic T cell (CD+8) encounters a peptide fragment that has a complementary shape to its receptor, it will bind to the MHC-peptide complex and secrete cytotoxic molecules that enter the infected cell and kill it, effectively ending production. more viral particles. As a result, MHC class I proteins work to present the types of proteins that are synthesized within a cell, so that they can be monitored by lymphocytes to destroy cells that produce unknown proteins, i.e. tumor cells or virus-infected cells.

MHC II glycoproteins are present only on macrophages, dendritic cells and B cells. All three of these cell types are capable of phagocytosis and their function is to phagocytose antigens that come from outside the cell, for example example on bacteria. After the exogenous

Antigens are broken down, the resulting peptide fragments are bound to MHC II molecules and presented on the cell surface. These cells will typically migrate to nearby lymph nodes where helper T cells with receptors that match the antigen have more opportunities to encounter the antigen and bind to it. When this happens, helper T cell lymphocytes become activated and begin to release cytokines that attract other cells to the area of ​​infection to destroy infectious agents with that antigenic material. B cells can also phagocytose foreign antigens, break them down, and expose the resulting peptides to MHC II molecules on their surface. If a helper T cell binds to a peptide fragment on the surface of a B cell, it stimulates the B cell to divide repeatedly and differentiate into plasma cells that produce antibodies against the antigenic material.

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The innate immune system is activated by PAMPs or, in the case of natural killer cells, by the absence of MHC class I molecules on the surface of the cell, but the adaptive immune system is activated by very specific molecular forms, generally called antigens. The illustration on the left is a representation of an influenza virus, which consists of an outer shell of hemagglutinin and neuraminidase proteins and eight strands of RNA in its core. The hemagglutinin and neuraminidase proteins are potential antigens, but there are only specific portions of these molecules that could be “recognized” by our immune system. The illustration on the right is a zoomed-in image of a hemagglutinin protein, and the portion of the molecule circled in red may represent a specific shape, i.e., an epitope

When the influenza virus infects our cells (for example, the epithelial cells of the nose and throat), its protein coat disintegrates, and the viral RNA uses our ribosomes and substrates to produce more viral proteins and more copies of his DNA. However, as noted above, samples of internally synthesized proteins (including viral proteins) are broken down into proteasomes, and the fragments are complexed with MHC Class I molecules in the endoplasmic reticulum. The MHC class I and attached fragments are then inserted into the cell membrane where the fragments are “presented” to cells of the immune system. These events are represented in the following figure. Helper T cells with matching receptors would activate and recruit additional lymphocytes, while cytotoxic T cells with matching receptors would bind to the cell and secrete cytotoxic molecules that enter the infected cell and kill it, effectively ending the production of more virus particles.

In the image below the virus binds to a human epithelial cell and internalizes. It then sheds its protein coat and begins to replicate viral RNA and proteins using cell organelles and substrates. Some viral proteins are transported from the endoplasmic reticulum to the proteasomes which break them into fragments that are bound to MHC Class I molecules. These are then transported to the cell membrane and inserted with the protein fragments “presented” on the outside of the cell where the T cells with corresponding receptors can bind to the fragments and become activated.

Helper T Cells Function In The Adaptive Immune System Activation

B cells can become activated by direct contact with a pathogen or foreign protein if they have a receptor complementary to an epitope of the foreign agent. The helper T cells that have been activated by antigen presentation will further stimulate the activated B cell to replicate over and over and transform into a large clone of plasma cells that produce antibodies specific to that epitope. These antibodies are widely distributed in the circulation and can bind to epitopes, marking foreign agents to facilitate identification and destruction by phagocytic cells. The following image shows an antibody binding to a specific epitope on two virus particles. Keep in mind, however, that antibodies can similarly participate in defense against any agent or substance that has matching epitopes.

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The following graphic illustrates the sequence of events that occur during hepatitis A virus (HAV) infection. Note, first, that the presence of the virus in the blood (viremia) and stool occurs well before symptoms appear, making it easy for a victim to transmit the virus to others. Also, note that the levels of IgM antibodies in the blood increase early and then begin to decrease. IgG levels increase a little later, but persist for a much longer time. By measuring the titers (concentrations) of IgM and IgG antibodies against HAV, it is possible to determine whether an individual was recently infected or was infected some time ago. This information could be important in determining whether a particular food handler, for example, was responsible for a hepatitis A outbreak. The adaptive, or acquired, immune response takes days or even weeks to establish, much longer than the innate response; however, adaptive immunity is more specific to pathogens and has memory. Adaptive immunity is immunity that occurs after exposure to an antigen from a pathogen or vaccination. This part of the immune system is activated when the innate immune response is insufficient to control an infection. In fact, without the information coming from the innate immune system, the adaptive response could not be mobilized. There are two types of adaptive responses: the cell-mediated immune response, which is carried out by T cells, and the humoral immune response, which is controlled by activated B cells and antibodies. Activated T cells and B cells that are specific to the molecular structures of the pathogen proliferate and attack the invading pathogen. Their attack can directly kill pathogens or secrete antibodies that enhance pathogen phagocytosis and terminate infection. Adaptive immunity also involves a memory to provide the host with long-term protection from reinfection with the same type of pathogen; upon re-exposure, this memory will facilitate an efficient and rapid response.

Unlike the NK cells of the innate immune system, B cells (B lymphocytes) are a type of white blood cell that give rise to antibodies, while T cells (T lymphocytes) are a type of white blood cell that play an important role in the immune response. T cells are a key component in the cell-mediated response, the specific immune response that uses T cells to neutralize cells that have been infected by viruses and some bacteria. There are three types of T cells: cytotoxic, helper, and suppressor T cells. Cytotoxic T cells destroy virus-infected cells in the cell-mediated immune response, and helper T cells play a role in activating both antibody and cell-mediated immune responses. Suppressor T cells deactivate T cells and B cells when necessary, thus preventing the immune response from becoming too intense.

An antigen is a foreign or “non-self” macromolecule that reacts with cells of the immune system. Not all antigens will elicit a response. For example, individuals produce countless “self” antigens and are constantly exposed to harmless foreign antigens, such as food proteins, pollen,

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