ELISA

Enzyme-Linked Immunosorbent Assay (ELISA) is a technique whereby antibodies are used for detecting and quantifying the prescence of a specific protein or antigen. It is extremely sensitive and can therfore detect very slight changes in the abundance of a molecule. The antibodies used in the technique have been modified such that they are linked with an enzyme that reacts with a particular substrate and results in a change in the colour of that substrate. The technique relies on the formation of an antibody-antigen complex when the protein comes in contact with the antibody within the reagent. This process can either be direct or indirect and they both undergo various different steps in order to reach the final result. A direct ELISA uses a single antibody that is complementary to the antigen you are testing for where as an indirect ELISA uses two different antibodies. 

Direct ELISA

Antigens are initially bound to the inside of a well and then a complementary antibody with an attached enzyme is added. If the antigen of interest is present in the sample the antibodies will bind thus forming a complex. The well is then washed to remove any unbound antibody and a substrate solution is added in order to allow a colour change to occur if the detection antibody is present as the enzyme and substrate will react.