The Structure And Function Of Antibodies
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Published: Tue, 18 Apr 2017
There is a type of white blood cell called plasma cells which are produced in the bone marrow as B cells then mature into plasma cells, these produce antibodies. Antibodies are either attached to cell surface membranes or secreted as soluble glycoproteins. Antibodies are large Y-shaped proteins which the immune system uses to neutralise and lead to the elimination of foreign bodies. Antibodies are glycoproteins, due to carbohydrates binding to amino acid residues on the polypeptides; these are composed of four polypeptide chains, of which, two heavy chains and two light chains to form the complete antibody. There are small regions at the tip of the antibody called the antigen binding sites; this region is hugely diverse due to random genetic mutations leading to amino acid chain variations causing a hyper variable region that allows it to bind to many different antigens.
Adaptive immunity is the immune response that involves antibodies. It is undeveloped at birth, and is the response of the lymphocytes to specific antigens.
Antibodies are heavy globular plasma proteins that belong to the family of proteins, immunoglobins. They have sugar chains attached to some of their amino acids making them glycoproteins. Each of their heavy chains has two regions; the constant region (carboxyl-terminal end) for biological effector functions and the variable region (amino-terminal end) for antigen recognition. The light and heavy chains forming the antibody have inter and intra chain disulphide bridges which hold the chains together, the quantity of bonds varies between different antibody molecules. They have a hinge region where the arms of the antibody molecule form a Y-shape; it is named the hinge region due to segmental flexibility at this point. Antibodies have a massively variable antigen binding site due to the different heavy and light chain amino acid configurations.
After birth the only antibodies present in the body are the ones passed over by passive immunization from the mother. Early active immune system antibodies develop in the first few years of life.
The main function of each antibody is to specifically bind to one or few similar antigens (foreign molecules). The structure of antibodies relates to the three main functions; activity, versatility and specificity. Antibodies prevent pathogens from damaging or entering cells by binding to them. Antibodies stimulate macrophages to engage in the removal of pathogens and also stimulate other immune responses. They bind to various cells such as phagocytes, lymphocytes, platelets etc. this binding leads to the activation of these cells to perform immune functions such as antibody production and phagocytosis. Antibodies can also bind together when theyâ€™re bound to pathogens, linking them together and stopping the pathogens from moving or causing damage.
The function of an antibody binding to an antigen is provided by the structure of the variable region which has the antigen-binding site (known as the Fragment antigen-binding fragment made from one constant and one variable region); the variable amino acid configuration allows a diverse possibility of specific antibodies to bind with antigens found on foreign bodies. The Fragment crystallisable region at the base of the antibody triggers the appropriate immune response for the situation, for example clumping together (where the Fab fragment joins with the Fc region of another antibody) or triggering the release of histamine in an allergic reaction.
There are five different antibody isotypes in humans; IgG, IgA, IgM, IgD, and IgE. IgG is the main antibody in the blood however it can move throughout the bodyâ€™s tissue. It forms the majority of the active immune antibody response to pathogens. It is also able to cross the placenta during pregnancy, passing on passive immunisation from the mother to the developing foetus. IgA is present in bodily fluids in entrances to the body, such as tears, breast milk, and saliva and also in the respiratory tract, urogenital tract and digestive tract, and its function is to prevent colonisation from pathogens. IgM is either present on B cell surfaces or in a soluble secreted form (in which is the largest antibody due to its pentamer form) in the blood and it is involved in the early immune response and can kill pathogens. IgD is the antigen receptor on B cells not already exposed to antigens. IgE is involved in the allergic response to foreign bodies and releases histamine when bound to allergens. The B cell will produce these various isotypes at different stages of its development.
Antibodies are secreted by a type of white blood cell called a plasma cell. Antibodies can occur in two physical forms, a soluble form that is secreted from the cell, and a membrane-bound form that is attached to the surface of a B cell and is referred to as the B cell receptor (BCR). The BCR is only found on the surface of B cells and facilitates the activation of these cells and their subsequent differentiation into either antibody factories called plasma cells, or memory B cells that will survive in the body and remember that same antigen so the B cells can respond faster upon future exposure. In most cases, interaction of the B cell with a T helper cell is necessary to produce full activation of the B cell and, therefore, antibody generation following antigen binding. Soluble antibodies are released into the blood and tissue fluids, as well as many secretions to continue to survey for invading microorganisms.
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