A secondary antibody is an antibody that specifically recognizes and binds to the primary antibody, rather than to the antigen. Secondary antibodies are commonly used in laboratory techniques such as Western blotting, ELISA, immunohistochemistry, and lateral-flow assay to detect the presence or expression of antigens of interest in a sample. They are usually conjugated with a detection molecule, such as a fluorescent dye, enzyme, or colloidal gold, that allows for visualization or quantification of the antigen-antibody interaction. The use of secondary antibodies enables the detection of multiple antigens in the same sample, as well as the characterization of the primary antibody specificity and cross-reactivity.