A labeled antibody is required in:
Western blotting, synonymously called immunoblotting, relies fundamentally on specific antigen-antibody interaction for protein identification and quantification. Workflow involves SDS-PAGE separation, electrotransfer to membrane, blocking, incubation with unlabeled primary antibody against epitope, followed by labeled secondary antibody conjugated to horseradish peroxidase, alkaline phosphatase, fluorophore, or bioluminescent enzyme. Signal originates from label attached to antibody. Polymerase chain reaction amplifies DNA using oligonucleotide primers and does not involve antibodies. Southern and Northern blotting detect nucleic acids through labeled complementary probes, not immunoglobulins, highlighting unique requirement for labeled antibody in immunoblotting.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.