Hemagglutination, or haemagglutination, is a specific form of agglutination that involves red blood cells (RBCs).
It has two common uses in the laboratory: blood typing and the quantification of virus dilutions in a haemagglutination assay.
A direct Coombs test is also done on the patient's blood sample in case there are any confounding antibodies.
By serially diluting a virus suspension into an assay tray (a series of wells of uniform volume) and adding a standard amount of blood cells, an estimation of the number of virus particles can be made.
By using a standard amount of virus, a standard amount of blood cells, and serially diluting the antiserum, one can identify the concentration of the antiserum (the greatest dilution which inhibits hemagglutination).