How is the hemagglutination inhibition test used to identify influenza strains?

The hemagglutination inhibition (HAI) test is a serological test used to identify influenza strains by measuring the ability of antibodies to inhibit the hemagglutination reaction between influenza viruses and red blood cells. Here’s how it works:

1. Virus preparation: Influenza virus is grown in culture and inactivated.
2. Red blood cell preparation: Red blood cells are collected and treated to enhance their sensitivity.
3. Antibody preparation: Serum samples containing antibodies against influenza are collected from patients or animals.
4. Hemagglutination reaction: The inactivated virus is mixed with red blood cells, causing hemagglutination (clumping of red blood cells).
5. Hemagglutination inhibition: The serum sample is added to the virus-red blood cell mixture. If antibodies are present, they bind to the virus, preventing hemagglutination.
6. Titration: The serum sample is serially diluted, and the HAI reaction is performed at each dilution.
7. Endpoint determination: The highest dilution at which hemagglutination is inhibited is the endpoint.