I have been reading up on the EMIT technique, but I am not sure that I fully understand its principle. When the antibody binds the antigen (which is covalently joined to the assay enzyme), the enzyme is inactive. Is that just because the antibody blocks access to (occludes) the active site of the enzyme? How common is it to perform this technique with a standard curve (sometimes said to be semi-quantitatively) versus without a standard curve (qualitatively)?
From a 1975 paper by AF Rosenthal et al. "The antibody bound to the labeled drug then sterically inhibits the enzyme activity; the higher the concentration of unlabeled drug, the more competition there is for binding sites on the antibody, and thus more release of enzymatic activity, which serves as the indicator for the immunologic competition."