The immune response to influenza virus. 3. Changes in the avidity and specificity of early IgM and IgG antibodies.

Serum samples taken from rabbits 5 days after vaccination with SW influenza virus by the intravenous route contained high levels of IgM antibodies. IgG antibodies were either not detected or were present at very low levels. By the 10th day after vaccination both IgM and IgG antibodies were present in the serum. The early IgM antibodies were of high avidity while the early IgG antibodies were of very low avidity. The presence of low avidity IgG antibodies in whole serum caused a decrease in the average avidity of the antibodies in whole serum from the 5th to the 10th day post vaccination. The avidity of the IgM antibodies remained fairly constant for the first 20 days of the immune response but a slight increase was detected after secondary vaccination. The avidity of the early IgG antibodies increased during the test period of 20 days. The early IgM and IgG antibodies were heterogeneous with respect to avidity. The highly avid IgM antibodies showed high cross-reactivity with related influenza viruses, i.e. they were of low specificity. The early IgG antibodies that were of low avidity cross-reacted with only one other influenza virus out of the four tested, i.e. they were more specific; as the avidity of the IgG antibodies increased so did their cross-reactivity.