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Antigen presenting cells APCs are quintessential in the facilitation and regulation of the immune response. The functions performed by the APC's are just a few of the various mechanisms employed by the immune system to control and eliminate invading pathogens. In general all cells are termed antigen presenting cells as they contain a major histocompatibility complex (MHC) on their surface. The MHC molecules allow the APC to carry out its highly important function in the processing and displaying of foreign antigen which T-cells recognise. There are two classes of MHC's, MHC class I and MHC class II which present different sets of antigens. Only a few cells display MHC class II molecules on their surface so are known as professional antigen presenting cell. All the remaining cells contain MHC class I thus are termed non- professional APC's. In addition to antigen presentation, APC's also provide co-stimulatory signals via B7.1 (CD80) and B7.2 (CD86). The individual functions carried out by MHC class II and MHC class I in the immune response will be discussed respectively.
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MHC class II molecules are constitutively expressed on professional APC's only (ref). Examples of these professional APC are Macrophages, dendritic cells and B cells. The process occurs when the antigen (derived from virus or bacteria) is taken up into the cell normally through endocytosis or phagocytosis .(http://books.google.com/books?id=Lxcj-PlALiIC&pg=PA109&dq=MHC&hl=en&ei=S1BuTICHOoXq4gaz6YDfCA&sa=X&oi=book_result&ct=result&resnum=4&ved=0CDwQ6AEwAw#v=onepage&q=MHC&f=false) PAGE 112!!
The antigen is processed and are presented by the MHC II molecules. These MHC class II molecules when bound to endogenous antigens are recognised by T cells through its CD4 receptor (CD4+ve). This is then targeted by CD4+ve T- cells (Th cell). The consequence of this is the generation or amplification of a specific immune response.
Dendritic cells are the key antigens presenting cells of the immune system. They are generated from progenitors in the bone marrow. They migrate into peripheral tissues through the blood stream. The immature dendritic cells lie and wait for pathogens entering the body, through sites of injury for example. Dendritic cells express various pattern recognition receptors that can recognise common features of many bacterial and fungal pathogens. Through these receptors they are able to bind to and phagocytose pathogens. When these receptors bind pathogens, they activate dendritic cells which then start to mature. In this process they migrate from the tissues and change their behaviour to stop phagocytosis and start expressing immune stimulatory molecules. The activated dendritic cell migrate from the tissue and into lymphatic vessels where lymphatic fluids drain through lymph nodes carrying the dendritic cells with it. T cells migrating through the lymph nodes inspect the dendritic cells for the presence of antigen. T cells that fail to recognise the antigen presented via the MHCII molecules on one dendritic cell carry on to inspect other dendritic cells and eventually return to the circulation. Different T cells have different variable portions on their receptors so will only bind to certain antigens presented...Each T-cell has a ceraint specificity to a combination of MHC II complex and a certain polypeptide (part of a virus). Th cells that do recognise their specific antigen binds through its T-cell receptor (TCR) become activated. Dendritic cells are the best cell at activating T-cells, particularly a naiive helper t cell. When the t cell becomes activated it both proliferate and differentiate into effector Th cells and memory Th cells which will be particularly useful in future infections by the same pathogen. The effector Th cell function in 'raising the alarm', it starts releasing molecules known as cytokines. There are many different kinds of cytokines, viewed as 'chemical alarm bells'. When Cytokines enter other activated immunological cells, it makes them get in gear, more active in their immune response. Cytokines tell activated cytotoxic T-cells to get in gear and also tells activated B cells to keep proliferating.
B lymphocytes (B cells) are cells from the bone marrow have protein complexes on their surfaces referred to as membrane bound antibodies (immunoglobulins). Each B-cell has one type of membrane bound antibody with its specific variable portion. Other B cells have a different membrane bound antibodies with a different variable portions. There are alot of different combinations of variable portions owing to the diverseness of the immune response to many ttypes of pathogens. Virus and vacteria, mutaing and evoluving, what the immune system has done diversity of variable portion...so many diffent combinations, , eventually...one B cell will bind to that type foreign pathogen via its epitope.. B cells become activated when the appropriate antigen binds. In some types of antigens, in order for the B cell to become activated..the services of Th cells are also required. These are known as t-cell dependent antigens (t cell dependent activation). This helps to direct the most appropriate immune response
That B cell that binds then becomes activated..this is witht he help of helper T cells.... The B cells then starts cloning itself and differentiate..
In the case of T-cell dependent antigensThe activate B cells to produce antibodies.
Effector t cells and antibody molecules then return to the circulation
CD4 t cells avtivate macrohphages to become more cytotoxic whilst antibodies...
Antigen presenting Macrophages activate helper t- cells with matching receptors. These in turn rendez vous with activated B cells. Triggered by this meeting, the helpers release chemicals which spur the selected B cell into rapid reproduction. Some B-cells become memory cells ready to respond to a later invasion by the same pathogen but most plasma cells which produce antibodies. Freely circulating in the body, antibodies doct with pathogens (opsonisation). AgglutinisaTION...through..variable portions... This neutralises them or marks them fro destruction by other immune cells i such as....