Practice question

Direct ELISA is simplest format where antigen from sample is adsorbed directly to polystyrene plate through passive hydrophobic interactions. After blocking, enzyme conjugated primary antibody specific to antigen is added, binding directly without secondary antibody. Substrate addition generates signal directly proportional to antigen present. It detects antigen not antibody, unlike indirect ELISA which detects antibodies. Only one antibody layer reduces steps but lowers amplification and specificity compared to sandwich formats. Its utility lies in measuring purified antigens or screening antibody specificity with enzyme labeled primary antibody.