2.2 IDENTIFICATION OF VIRUSES USING ANTIBODIES (SEROLOGY)

 Antibodies are proteins produced by the immune system of higher vertebrates in response to foreign materials (antigens) which those cells encounter. Such antibodies have a region that recognizes and binds specifically to that same antigen. Antibodies are secreted into the body fluids and are most easily obtained from blood. Blood is allowed to clot and antibodies remain in the fluid part (serum) which remains after clotting has removed cells and clotting proteins. This is then known as an antiserum. The principle of identifying infectious virus by using an antibody of known specificity is shown in Fig. 2.6. If the antibody recognizes and binds to the virus, virus infectivity will be inhibited (Fig. 2.6, top line). Infectivity is only one of several virus properties that can be affected by antibody binding, and hence can be monitored in this type of assay. Another is inhibition of the agglutination of red blood cells by virus. This is a property of some viruses, like the influenza viruses, that attach to molecules on the surface of the red blood cell (RBC). At a certain virus to cell ratio the RBCs are linked together by virus and the cells are agglutinated (clumped together). This has nothing to do with infectivity, and when infectivity of a virus has been deliberately inactivated, that virus can still agglutinate RBCs efficiently, providing that its surface properties are unimpaired. A quantitative hemagglutination test can be devised by making dilutions of virus in a suitable tray and then adding a standard amount of RBCs to each well (Fig. 2.7). The amount of virus present is estimated as the dilution at which the virus causes 50% agglutination. This test has the advantage of speed – it takes just 30 minutes, compared with an average of 3 days for a plaque assay. However, it is insensitive;


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