HI Typing Assay

HI Assay overview Biorender

The Haemagglutination Inhibition (HI) Assay is used to identify and type new and existing virus isolates, using antisera prepared against known standard viruses.

As mentioned in the 'About influenza' section, influenza viruses are covered in surface proteins called haemagglutinin, which bind specifically to sialic acid-containing receptors. These receptors are found on the plasma membrane of certain cells, such as red blood cells (RBCs). When RBCs are mixed with influenza virus at the appropriate ratio, the virus binds these receptors (and therefore the RBCs), forming a lattice structure. This process is called haemagglutination.

Under normal conditions (without virus), when suspended RBCs are placed inside the well of an assay plate, the cells will slowly settle to the bottom of the well over time. In the presence of influenza virus, however, haemagglutination will alter how the RBCs settle.

Addition of antibodies/antisera (prepared against antigenic determinants like haemagglutinin) to this mixture interferes with, or inhibits the binding of viral haemagglutinin to these receptors on the RBC membrane.

Reagents for typing influenza isolates (2019 onward)

In the past, the Centre has prepared and made available a kit of influenza reagents for laboratories in the Asia-Pacific region to conduct their own initial identification test on virus isolates. The 2019 Influenza Reagent Kit was ready in June 2019 and a large number of kits were sent out at that time.

Due to the lack of demand for the Centre’s kits, this activity will cease after 2019. Leftover 2019 kits that are still within the expiry date will be supplied to laboratories requesting them in 2020 but no new kits will be made. Laboratories requesting kits for 2020 and beyond will need to do so via the International Reagent Resource, who supply a similar kit provided by the Centers for Disease Control and Prevention, Atlanta, GA, USA.

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