An antibody is a protein produced by lymphocytes of the immune system in response to the presence of a foreign antigen in the system. It usually binds to the foreign antigen and causes it to precipitate from the circulation (Ross, Romrell and Kaye 7). Antibodies can be purified in the laboratory and conjugated (chemically bonded) to a variety of substances, including a fluorescent dye such as fluorescein. This conjugated antibody can then be applied to sections of tissue that have been lightly fixed or frozen so that the location of the antigen in the tissue being examined can be visualized using a fluorescence microscope. This is known as immunohistochemistry or immunocytochemistry.
The antibodies are produced by isolating a protein from one species, such as a rat, and injecting it into another species, such as a rabbit. The protein will then stimulate the rabbit to make antibodies against it, which can then be harvested from the rabbit's blood where they circulate, conjugated to fluorescent dyes, and then used to localize that particular rat antigen in tissues suspected of containing it. If the antigen is present, the antibody part of the conjugate would bind to it and could then be visualized under the fluorescent microscope because of the fluorescent dye it contains (7-8). The antibodies can also be conjugated to gold or ferritin so that they can be used to stain tissues to be examined under the electron microscope.
Enzyme histochemistry can be combined with traditional immunohistochemical methods to achieve greater amplification of the localization reaction than is possible with fluorescent dyes such as fluorescein (8). In enzyme histochemistry, the enzyme horseradish peroxidase is conjugated to the primary antibody, and once the antigen-antibody reaction has occurred, the histochemical process for visualizing peroxidase activity is then carried out to demonstrate the location of the antigen-antibody complex in ...