What are the different types of ELISA?

Direct: the use of only a primary antibody conjugated with a substrate. indirect: the use of a primary antibody and a secondary antibody conjugated with a substrate. sandwich: the use of a capture antibody that "captures" the antigen as it is added, as well as a primary antibody and a secondary antibody conjugated with a substrate that bind on top of this to visualise the reaction. competitive: the sample antigen competes with a reference antigen in binding to the capture antibody, which is indicative of the concentration of the sample antigen.