Enzyme-linked immunosorbent assay (ELISA) as a method that revolutionized medicine discovered by Eva Engvall and Peter Perlman in 1971. 
ELISA is a labeled immunoassay that is considered the gold standard of immunoassays. This immunological test is very sensitive and is commonly used to measure antibodies, antigens, proteins, glycoproteins, and hormones in biological samples. Some examples include diagnosis of HIV infection, pregnancy tests, and measurement of cytokines or soluble receptors in cell supernatant or serum. The detection of these products is accomplished by complexing antibodies and antigens to produce a measurable result. ELISA assays are generally carried out in 96 well plates, allowing multiple samples to be measured in a single experiment.
The four main types of ELISAs are indirect, direct, sandwich, and competitive.
The most straightforward version of these assays is the direct ELISA, a test capable of identifying antigens in a sample by optimizing the formation of antigen-antibody complexes. In a direct ELISA, the primary detection antibody binds directly to the protein of interest. Next, the plate is rewashed to remove any unbound antibody and followed by the addition of a substrate.

Read more about the other types of ELISA