The description of ELISAs is similar to that described for the radioimmunoassay, but in place of radioactively labeled antibody or antigen, various fluorochromes have been substituted in place of the radioactive label. In the presence of an appropriate substrate, the fluorochrome-labeled antibody is activated to produce a given color, and the intensity of the color is read on a spectrophotometer using 450-nm filter. By keeping the known antigen constant and diluting the serum to be tested, one can produce a curve of decreasing optical density readings, thereby indicating the amount of antibody in a given serum when compared with a standard control.
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