The major use of Enzyme Linked Immunosorbent Assay (ELISA) is to titrate antibodies against specific antigens (Crowther, 1995). This method involves an enzyme, which reacts with a colourless substrate to form a coloured product, is covalently linked to a specific antibody that recognises a specific antigen. If the antigen is present, the antibody-enzyme complex will bind to it, and the enzyme component of the antibody-enzyme complex will catalyse the reaction which results in a coloured product (Berg et al, 2002). The presence of the coloured product indicates that an antigen is present. The ELISA, which is rapid and convenient, can detect less than a nanogram (10-9 g) of a protein and can be performed with either polyclonal antibodies, which are antibodies that can recognise multiple epitopes on any one antigen or monoclonal antibodies, which are antibodies that can only detect one epitope on the antigen (Guo et al, 2006). However, the use of monoclonal antibodies gives more reliable and accurate results due to their specificity as monoclonal antibodies are very effective at acting as the primary antibody in an assay, or for detecting antigens within a tissue. Also, they will usually result in reduced background staining compared to that of polyclonal antibodies.
Indirect ELISA and direct ELISA Different types of ELISA’s have been developed with modifications to the basic steps. The key step with the ELISA assay is the direct or indirect detection of antigen by