Figure 1: Diagrammatic representation of a blocking ELISA. Y = antibody in test serum. e-Y = detector antibody conjugated to a marker enzyme.
A competitive ELISA works on the same principle but in this case the test serum and the competing antibody (MAb or polyclonal serum specific for the antigen) are added as a mixture to the wells on the plate. They compete for binding sites on the antigen. In the simplest versions of the test the competing antibody is conjugated to a marker enzyme (
). The plate is then washed and substrate added in the normal way. The percentage of inhibition of colour formation is calculated.