Western blot is used to detect specific proteins in tissue or cell extracts. It involves separating proteins by electrophoresis, transferring them to a membrane, and using antibodies to identify a target protein. ELISA detects specific antigens or antibodies and is used for diseases screening like HIV, syphilis, and Lyme disease. PCR amplifies specific DNA sequences and is used to detect pathogens and for disease diagnosis. Plasmid fingerprinting identifies bacteria by comparing their plasmid profiles.