Centers for Disease Control and Prevention have determined that some adults have serum cross-reactive antibodies to the new influenza H1N1 virus. One of the techniques used to reach this conclusion is the hemagglutination inhibition (HI) assay. How does this assay work?
To understand the HI assay, we must discuss the hemagglutination assay. Influenza virus particles have an envelope protein called the hemagglutinin, or HA, which binds to sialic acid receptors on cells. The virus will also bind to erythrocytes (red blood cells), causing the formation of a lattice. This property is called hemagglutination, and is the basis of a rapid assay to determine levels of influenza virus present in a sample. To conduct the assay, two-fold serial dilutions of a virus are prepared, mixed with a specific amount of red blood cells, and added to the wells of a plastic tray. The red blood cells that are not bound by influenza virus sink to the bottom of a well and form a button. The red blood cells that are attached to virus particles form a lattice that coats the well. The assay can be performed within 30 minutes, and is therefore a quick indicator of the relative quantities of virus particles.