In a home pregnancy test, monoclonal antibodies are used to detect the hormone hCG. A positive result is indicated by a colored line. This occurs because:

the hCG directly reacts with a dye in the test strip
the monoclonal antibody simply changes color when it detects hCG
the patient's own antibodies against hCG are detected by the test
the hCG antigen binds to both a mobile color-tagged antibody and an immobile capture antibody, concentrating the color

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