In competitive ELISA, unlabeled antibody is incubated in the presence of its antigen. Then these bound antibody/antigen complexes are then added to an antigen coated well. After washing, unbound antibodies are removed. The more analytes in the sample, the less antibodies will be able to bind to antigens in the well. The signal is then detected using labeled secondary antibodies and the decrease in signal is compared to a control. The major advantage of a competitive ELISA is the ability to use crude or impure samples and still selectively bind any antigen that may be present.