The illustrations on Wikipedia don't explain. If antigen excess and high analyte concentration saturate capture and detection antibodies, then wouldn't just fewer immune complexes be agglutinated? How does the high analyte concentration thwart sandwlich ELISAs from forming at all?
Mechanism and in vitro importance
Version with excess antibodies
In an agglutination test, a person's serum (which contains antibodies) is added to a test tube, which contains a particular antigen. If the antibodies agglutinate with the antigen to form immune complexes, then the test is interpreted as positive. However, if too many antibodies are present that can bind to the antigen, then the antigenic sites are coated by antibodies, and few or no antibodies directed toward the pathogen are able to bind more than one antigenic particle. Since the antibodies do not bridge between antigens, no agglutination occurs. Because no agglutination occurs, the test is interpreted as negative. In this case, the result is a false negative. The range of relatively high antibody concentrations within which no reaction occurs is called the prozone.
Version with excess antigens
The effect can also occur because of antigen excess, when both the capture and detection antibodies become saturated by the high analyte concentration. In this case, no sandwich can be formed by the capturing antibody, the antigen and the detection antibody. [Bolding mine] In this case, free antigen is in competition with captured antigen for detection antibody binding. Sequential addition of antigen and antibody, paired with stringent washing, can prevent the effect, as can increasing the relative concentration of antibody to antigen, thereby mediating the effect.[_citation needed_]