#Antigen_Antibody_Reaction
#Dr_Najeeb
#Microbiology


Antigen–Antibody Reactions
Antigen–antibody reactions are a mainstay for the rapid detection of proteins. Antibodies recognize proteins based on their structure as well as content, and can be very specific, binding to only a small part of an antigen (known as the epitope), and discriminating between highly similar epitopes. In nature, if a host is exposed to an antigen, the host will develop an array of antibodies that each bind to a separate epitope of the antigen. These antibodies will vary in specificity. Homogeneous, highly sensitive and specific antibiotics have been developed and propagated in the laboratory. These are known as monoclonal antibodies. Because of their high sensitivity and specificity there are multiple applications for antibodies in clinical and research settings, mostly focused on diagnosis, although there are therapeutic uses, such as passive immunization for botulism, rabies, and other diseases.
The most common application of antigen–antibody reactions is in diagnostics using an enzyme-linked immunosorbent assay (ELISA). For example, ELISA can be used to detect an antibody to HIV; this is known as indirect or sandwich ELISA. ELISA also can detect an antigen for human chorionic gonadotrophin(HCG), which is excreted in the urine of a pregnant woman; this is called direct ELISA. The target (either antibody or antigen) of interest is bound to a substrate. For a pregnancy test, the target is an antibody for HCG, and for an HIV test, an HIV antigen. The target can be on the bottom of a well in a plastic plate or some other substrate. After the test specimen is added, if the protein of interest is present, it will bind to the target. For indirect ELISA, a second antibody is added; this one binds to the specific antibody (Figure 5.13A). For direct ELISA, a second antibody is also added, but this is to the target antigen (Figure 5.13B). The second antibody is labeled with a reporter enzyme that changes color when its substrate is added. The last step is adding this substrate. For the over-the-counter pregnancy test, the entire test takes place on a single platform. As the urine moves through the platform by capillary action, it releases free antibodies and the reporter enzyme. If HCG is present in the urine, it binds to antibodies fixed to the platform; the antibody with a reporter enzyme binds to the bound HCG. Bound HCG is also fixed to the platform as a control. A positive test shows two lines, the control result and that from HCG present in the urine.
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