Hemagglutination inhibition (HI or HAI) is used to determine relative concentrations of viruses, bacteria, or antibodies invented by American virologist George Hearst in 1941. This experimental method utilizes surface proteins of various viruses (such as hemagglutinin (HA) of influenza virus) to agglutinate multiple types of red blood cells (RBC). The reaction of viral hemagglutinin with red blood cells produces a lattice of aggregated cells. However, if the serum of a person infected with the virus is mixed with RBC and the virus, RBC will not agglutinate. This phenomenon is called hemagglutination inhibition. Antibodies present in the infected person's serum neutralize the virus, which leads to a positive result. If the patient's serum does not contain antibodies against the surface protein of the test virus, there will be a hemagglutination reaction because the surface molecule is free for the hemagglutinin RBC (negative result).
HAI is closely related to HA analysis, mainly through antiviral antibodies as "inhibitors" that interfere with the interaction of viruses and RBCs. The purpose is to characterize the concentration of antibodies in antiserum or other samples containing antibodies. HAI assay is usually performed by creating antiserum dilutions serially in each well of a 96-well microtiter plate. A standard amount of virus or bacteria will be added to each well, and the mixture will be incubated at room temperature. It is necessary to set up a negative control without adding virus for each sample. During the incubation, the antibody will bind to the virus particles. If the antibody concentration and its binding affinity are sufficiently high, the blood coagulation reaction can be effectively blocked.
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