Expand ELISA. On what principle is ELISA test based? List two ways by which an infection can be detected by this test.

ELISA or Enzyme-Linked Immunosorbent Assay is an immunoassay technique used for the detection of diseases. ELISA is based on the principle that specific antibodies bind to their target antigen i.e it is based on the principle of antigen-antibody interaction. Only when the interaction occurs the substrate will be able to bind to the enzyme and substrate conversion will be seen, thus giving a positive result.
Two ways by which the infection can be detected by this method are -
i) Direct ELISA: in which the sample is first added and then the enzyme-conjugated antibody is added. If an interaction occurs reaction will be seen on substrate addition.
ii) sandwich ELISA- in which the sample is sandwiched between two layers of antibodies and if interaction occurs substrate reaction will be seen on addition of a conjugated antibody and substrate.HIV and rotavirus infection can be detected by this technique.