ELISA was first described in 1971, and since then has become a more and more important technique in
diagnostic virology. ELISA has replaced a number of more cumbersome and time consuming "classical" serological techniques, and has also widened the scope of the
detection methods for viruses and their related markers of infection.
When a foreign particle enters the body, the body tries to synthesize antibodies(defence particles) , which combine with the antigen making it
susceptible to destruction or phagocytosis.
The same interaction of antigen and antibody outside the body--in the laboratory--can be used to determine whether a patient has an infectious or an
autoimmune disease. The test measures whether a specific antibody associated with an illness can be found in a patient's blood. A positive result
indicates that the antibody is there and implies that the person has encountered a particular disease.
A portion of serum possibly containing the antibody is allowed to react with the target antigen. A correct match causes the antigen and antibody to bind together.
Detection becomes possible when a second antibody is added. This antibody is prepared from the serum of an animal injected previously with human
antibody; the human antibody in this case serves as an antigen and the animal thus produces an antibody against the human antibody. Once isolated,
the second antibody can be chemically linked to a system that can produce a detectable signal.
In ELISA the label that is added to produce a detectable signal is an enzyme and therefore it is called enzyme linked immunosorbent assay.
The basic principles of the ELISA test are as follows:
- Antigens solubilised in an appropriate buffer can be coated on a plastic surface, like polystyrene. This may be directly or via an antibody.
When serum is added, antibodies can attach to the antigen on the solid phase.(Antigen-antibody binding)
- The presence or absence of these antibodies can be demonstrated with the help of anti-human immunoglobulin conjugate (indirect method) or with
conjugate specific against the appropriate antigen (direct method) respectively. The antibodies are conjugated to an enzyme, for example
- Adding a substrate, like HRPO, will detect the amount of bound conjugate by a degree of colour produced, which can be quantified. ELISA can
also be used for detection of antigens by using specific antibody on the solid phase. Adding an enzyme-linked antibody and a substrate leads
to colour production in proportion to the amount of antigen present. Commercially produced ELISA kits are now available for a wide range of
viral antigens and antibodies, including IgM antibodies.