ELISA is an immunological technique which is used to detect the presence of an antibody or an antigen in a sample. A known amount of antigen is coated on a solid phase like that in the well of a polystyrene microtitre plate and sample containing specific antibodies is added in the wells to allow antigen-antibody binding. The immune complex thus formed is linked to an enzyme, which acts on a substrate to give a coloured reaction which is detected using ultramodern ELISA plate readers. Since ELISA technique utilizes enzyme labels to produce detectable signals, it is given the name Enzyme-Linked Immunosorbent Assay.