hemagglutinating

Hemagglutinating describes the ability to cause hemagglutination, the clumping of red blood cells. This occurs when a molecule or particle binds to antigens on the surface of multiple red blood cells, cross-linking them into a lattice. The process can be mediated by antibodies, by viral surface proteins called hemagglutinins, or by lectins and other agglutinins produced by some bacteria and fungi. In virology, many enveloped viruses express hemagglutinin that binds to sialic acid residues on red cell membranes, leading to visible agglutination. The influenza virus is a classic example; the strength of hemagglutination depends on the virus subtype and red cell source.

In laboratory settings, hemagglutination is used in assays to detect viruses, to measure viral or antibody titers, and to characterize receptor specificities. A standard hemagglutination assay mixes red blood cells with serial dilutions of a test sample; nonagglutinated cells sediment to form a button at the bottom, while agglutinated cells remain diffuse. The highest dilution that still causes agglutination defines the titer. Hemagglutination inhibition assays further use specific antibodies to block the interaction between hemagglutinin and red cells, helping to quantify neutralizing antibodies.

Practical considerations include red cell type and receptor compatibility, temperature, pH, and ionic strength, all of which influence the assay outcome. Non-specific agglutination can occur, and some viruses require enzymatic treatment to expose receptors.

See also: hemagglutinin, hemagglutination inhibition assay, serology, blood typing.