Describe the two main immunoassay formats (competitive and sandwich) and indicate which is used for small molecules versus large antigens.

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Immunoassay formats differ in how binding events are converted into a readout, and the choice depends on the size and epitope structure of the target. In a competitive format, the sample analyte competes with a labeled version of itself for a limited number of antibody binding sites. Because those sites are finite, more analyte from the sample reduces how much labeled tracer can bind, so the detector signal falls as analyte concentration rises. This inverse relationship makes competitive assays ideal for small molecules that have only a single epitope and cannot bind two antibodies simultaneously.

In a sandwich format, the target antigen must present two distinct epitopes. One antibody captures the antigen, and a second, labeled detector antibody binds a different epitope, producing a signal proportional to how much antigen is present. More antigen leads to more detector binding and a higher signal, so this format suits larger antigens with multiple epitopes.

Thus, small molecules use competitive immunoassays with decreasing signal as analyte increases, while large antigens use sandwich immunoassays with increasing signal as analyte increases.