Hemagglutination assay
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The Hemagglutination Assay (HA) is a quantification of viruses.
Some viral families have surface or envelope proteins, that are able to agglutinate (stick to) human or animal Red blood cells (RBC) and bind to its N-acetylneuraminic acid. The RBC will form a type of lattice in this case.
In contrast to Plaque assay or LD50, HA does not give any measure of viral infectivity, because no virus replication is required in this assay.
It is an easy, simple and rapid method and can be applied to large amounts of samples.
The detailed conditions depend on the type of virus. Some viruses bind RBCs only at certain pH values, others at certain ionic strengths.
A virus dilution (eg. 2-fold from 1:4 to 1:4096) will be applied to a RBC dilution (eg. 0.1% to 0.7% in steps of 0.2%) for approx. 30 min, often at 4C, because else viruses with neuraminidase activity will detach the virus from the RBCs. Then the lattice forming parts will be counted and the titre calculated.
Virus concentration in virions per milliliter = 107 x HA titer, as was shown by H.B. Donald and A. Isaacs, J. Gen. Microbiol., 10, 457 (1954)
