Non-specific inhibitors
Some sera may contain substances other than antibodies that inhibit viral haemagglutinin. These substances are described as non-specific inhibitors and are rarely observed with chicken serum and Newcastle disease.
Natural agglutinins
Some chicken sera contain substances that will agglutinate chicken red blood cells. The settling pattern of the agglutinated cells is similar to that produced by Newcastle disease virus. These natural agglutinins are present in low concentration. The control serum well will indicate the presence of natural agglutinins.