Fish Diseases and Management

7.1.7.Antibody based diagnostic techniques

There are several techniques that are based on the antigen-antibody reactions, which can be effectively used in identifying different aetiological agents. Based on the requirement, the tests can be designed to detect either antigen or antibody in a sample. Immunohistochemical staining is a variation of the normal histology with the additional staining method using antiserum against specific pathogens. Viral or bacterial antigens are localized by an antibody raised against the pathogen and subsequently detected by visualizing the coloured product that is developed at the end of the reaction by light microscopy. Similarly, instead of colouring agent, if a fluorescent dye is used, it is referred to as immunofluorescence test. Thus the histological technique in combination with antigen-antibody reactions can be effectively used for the pathogen localization in vivo and also to delineate the histopathological changes that are associated with the infections.

Immunodiagnostics also helps to quantify the antigen or antibody present in a test sample by the type of technique used. The specificity and sensitivity of different types of immunological tests are varied. The specificity of immunological tests can be increased by the use of monoclonal antibodies over polyclonal antibody. The sensitivity of the reaction can also be enhanced by use of specific chemicals like avidin – biotin system as in case of Enzyme linked immunosorbent assay(ELISA) test. Earlier immunological tests were named based on the happenings to the antigens: viz. precipitation, agglutination, neutralisation, complement fixation etc. While recent tests are based on the reactions involved viz. enzyme – linked immunosorbent assay, immunodiffusion, immunofluorescence, immunoprecipitation etc.

ELISA is one of the efficient immunological diagnostic techniques, working on the principle of antigen antibody reactions. In ELISA, either an antigen or an antibody is linked with an enzyme label, which is used to indicate the presence of the formation of the antigen-antibody complex on a solid phase. This technique can be used to test either the presence of an antigen or an antibody in the test sample. There are different variations of ELISA technique. ELISA is a simple, versatile and highly sensitive test.