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An antibody is a protein complex used by the immune system to identify and neutralize foreign objects like bacteria and viruses. Each antibody recognizes a specific antigen unique to its target.
Antibodies are glycoproteins that are called immunoglobulins that are found in the blood and tissue fluids produced by cells of the immune system that bind to substances in the body that are recognized as foreign antigens. The forked end of the "Y" shaped antibody is known as the variable binding domain and stem the constant binding domain. Antibodies stick to pathogens by binding at the variable binding domain and work, in a variety of ways, to help eliminate the antigen that elicited their production by facilitating co-binding to the constant domain by the body's other immune cells. Some of the ways are independent to a particular class of immunoglobulins.
Immunoglobulins are grouped into five classes: IgG, IgA, IgM, IgD, and IgE. Differences in the heavy chain constant domains of the immunoglobulin determine its function and which of the following five classes it belongs to. Other immune cells partner with antibodies to eliminate pathogens depending on which IgG, IgA, IgM, IgD, and IgE constant binding domain receptors it can express on its surface.
Immunoglobulins are heavy plasma proteins, often with added sugar chains (see glycosylation) on N-terminal (all antibodies) and occasionally O-terminal (IgA1 and IgD) aminoacid residues. A crude estimation of immunoglobulin levels can be made by protein electrophoresis. Here the plasma proteins are separated into albumin, alpha-globulins (1 and 2), beta-globulins (1 and 2) and gamma-globulins according to weight. Immunoglobulins are all in the gamma region. In some disease states (myeloma) a very high concentration of one particular protein will show up as a monoclonal band.
Antibodies that recognize viruses can block these directly by their sheer size. The virus will be unable to dock to a cell and infect it, hindered by the antibody. Antibodies that recognize bacteria mark them for ingestion by macrophages. Together with the plasma component complement, antibodies can kill bacteria directly.
The way that antibodies work is by binding with the specific antigen for which the antibody is "designed". This formation of the antigen-antibody results in the stimulation of phagocytosis which is a procedure that cells engulf and destroy particles. An example of an antigen the can do this process is IgG antibodies that prevents the toxin harming the cell by sticking to the cell to destroy it. Antibodies are less effective if they are in low concentrations meaning that it's sometimes less effective in taking care of an already established infection such as viral infections. A viral infection can hide from an antibody so that it does not destroy it when it enters the cell but with bacterial infections they can be destroyed because they are outside of the cell.
Antibodies are effective in preventing any foreign antigens that go into the body. If an antibody can't take care of an already existing infection then it could be very effective in preventing an infection that is about to begin its process in targeting the cells.
When a macrophage ingests a pathogen, it attaches parts of its proteins to a class II MHC protein. This complex is moved to the outside of the cell membrane, where it can be recognized by a T lymphocyte, which compares it to similar structures on the cell membrane of a B lymphocyte. If it finds a matching pair, the T lymphocyte activates the B lymphocyte, which starts producing antibodies. A B lymphocyte can only produce antibodies against the structure it presents on its surface.
"Designed" monoclonal antibody therapy is already being employed in a number of diseases (including rheumatoid arthritis) and is a potential weapon against cancer.
In biochemistry, antibodies are used for immunological identification of proteins, using the Western blot method. A similar technique is used in ELISPOT assays, in which detection antibodies are used to detect cell secretions such as cytokines or antibodies.
Antibodies are also used to separate proteins (and anything bound to them) from the other molecules in a cell lysate. These purified antibodies are often produced by injecting the antigen into a small mammal, such as a mouse or rabbit. Blood isolated from these animals contains polyclonal antibodies -- multiple antibodies that stick to the same antigen. If the lymphocytes that produce the antibodies can be isolated and imortalized, then a monoclonal antibody can be obtained. Monoclonal antibodies have much greater specificity than polyclonal antibodies.
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