Multichannel pipette being used to add red blood cells (erythrocytes) to a microtiter plate in preparation for a haemagglutinin inhibition (HI) test. HI tests are used to characterise proteins on the surface of viruses known as antigens. Haemagglutinin (HA) is an antigen found on the surface of influenza (family Orthomyxoviridae) viruses. HA can bind red blood cells in a process called haemagglutination, causing them to form lattices. HI tests work by adding antibodies (type of protein) which bind to HA, inhibiting the process of haemagglutination. If one type of antibody inhibits HA in several strains of influenza (indicated by multiple small red dots), it suggests these strains of influenza have similar antigens.