Direct conjugate-labeled antibody detection

In a direct immunoassay the immobilized antibody binds to the corresponding antigen. The competitive immunoassay relies upon the competition of the analyte with a labelled analyte for antibody binding. These formats are widely used for high throughput affinity arrays. A sandwich immunoassay is based on the trapping or capture of the analyte by another antibody. In ELISA (enzyme linked immunosorbent assays) the second antibody is conjugated with an enzyme. The bound enzyme labelled antibody is detected by its ability to break down its substrate to a colored product. 

Multi-step technique (3) This is an indirect/direct method combining unlabeled primary antibodies with directly-conjugated antibodies. The method starts with staining the unlabeled antibody/antibodies with the appropriate detection system, but without performing the final enzymatic staining reaction. The tissue is blocked with normal serum from the host of the first primary antibody before the second, directly-labeled primary antibody is added. The staining ends with the two enzymatic reactions being performed sequentially. 

Direct non-competitive solid-phase enzyme immunoassays This approach is little used for quantitative investigations, but often in qualitative tests, e.g. for the detection of antigens on protein blots (Chapter 16) or in EIH (Chapter 17). For each epitope a corresponding labeled antibody is needed. The immobilized antigen (] = Ag) reacts directly with the conjugate (Ab E) ... 

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