Practice question

Sensitivity of immuno-detection is significantly influenced by labeling strategy. Direct labeling covalently attaches enzyme or fluorophore to primary antibody, yielding one reporter per antigen binding event with minimal background. Indirect labeling strategy incubates unlabeled primary antibody first, followed by several labeled secondary antibodies recognizing constant region of primary antibody. This recruitment amplifies signal because multiple secondary molecules bind single primary, enhancing detection of low-abundance targets and allowing common labeled secondary to serve many primaries cost-effectively. Tetramethylbenzidine is substrate oxidized for color development, not labeling modality, while biotin-streptavidin provides further amplification beyond basic indirect method.