Haemagglutination is visible macroscopically and is the basis of
haemagglutination tests to detect the presence of viral particles. The test does
not discriminate between viral particles that are infectious and particles that
are degraded and no longer able to infect cells. Both can cause the
agglutination of red blood cells.
Note that some other viruses and some bacteria will also agglutinate chicken
red blood cells. To demonstrate that the haemagglutinating agent is
Newcastle disease virus, it is necessary to use a specific Newcastle disease
virus antiserum to inhibit the haemagglutinating activity.
Substances that agglutinate red blood cells are referred to as
haemagglutinins