hemagglutination-inhibition assay Oliver He HI assay an immune response detection assay that detects specific antiviral antibodies in a test serum. The serum, usually a twofold dilution series, is mixed with a standard number, usually 4 to 8 HA units, of virus and incubated prior to the addition of a standard suspension of erythrocytes. The highest dilution of serum that inhibits hemagglutination is the HI titer of the serum. For example, influenza (AI) viruses agglutinate erythrocytes through the interaction of the virus surface glycoprotein, the hemagglutinin (HA), with receptors on the surface of the erythrocyte. Inhibition of the agglutination reaction by HA subtype-specific antisera is the basis of the hemagglutination-inhibition (HI) assay. assay immune response detection assay