Direct ELISA: the simplest type of ELISA which uses a labelled primary antibody that reacts directly with the antigen immobilized on the surface of a plate. This method is not widely used but is quite common for immunohistochemical staining of tissues and cells.
Indirect ELISA: similar to direct ELISA assays but detection requires a two-step process involving both a primary and a labeled secondary antibody against the host species of the primary. Primary antibodies bind to the immobilized antigen on the plate surface, and secondary antibody bind to the primary antibody which enables detection.
Sandwich ELISA: a sensitive and robust method using two layers of antibodies (capture and detection antibody) specific for different epitopes of the antigen. This requires the antigen to contain at least two antigenic sites capable of binding to both antibodies.
Competitive ELISA: this format is common when the antigen is small and has only one epitope, or antibody binding site. It involves a competitive binding process during which antigen from samples and a reference antigen compete for binding to a labelled capture antibody. As antigen concentration in a sample increases, signal intensity from the reference antigen decreases due to fewer free antibodies binding. Labelled antibodies are used instead of labelled antigens in some competitive ELISA kits.