ELISA (Enzyme-Linked Immunosorbent Assay) is a biochemical technique used to detect antibodies or antigens in a sample. There are three main types of ELISA: competitive ELISA where a labeled antigen competes for antibody binding sites; sandwich ELISA where antibodies coat a surface to detect a specific antigen; and indirect ELISA where a test antigen is coated on a surface and secondary antibodies are used to detect primary antibodies in a serum sample. The amount of color change produced corresponds to the concentration of antigen or antibody in the original sample.