Practice question

ELISA formats differ by what is immobilized and detected. Direct ELISA coats antigen and uses enzyme labeled primary antibody. Sandwich ELISA uses two antibodies to trap antigen. Competitive ELISA uses competition for signal reduction. Indirect ELISA coats antigen, commonly viral lysate, patient serum antibodies bind, then enzyme conjugated anti human immunoglobulin detects bound human antibody. HIV screening detects anti HIV antibodies in patient serum against coated viral antigens, so uses unlabeled primary from patient plus labeled secondary antibody, characteristic of indirect ELISA ensuring amplification and sensitivity for serology.