Cyclicity and memory in the humoral immune response of the marine toad Bufo marinus

Bufo marinus toads immunized with a single dose of 100 mug of polymerized flagellin (POL) produce a longlasting cyclic humoral response at 22 degrees C in which the serum antibody titers rise and fall within periods of 2-3 weeks. This seems to be regulated by the catabolism of immunoglobulins, since passive 125I-labeled IgM in the serum has a half-life of 17 days, corresponding to the cyclicity of the titers of active antibodies. In toads injected with between 10 ng and 10 mug antigen the serum antibody titer peaks between 4 and 7 weeks, depending on the antigen dose, and declines by the 9th week. At this time a second stimulation with an equal dose of POL induces a secondary response during which higher titers of antibodies rise faster than in the primary response. A dose of 10 mug POL stimulates an optimal primary response and a second equal dose, given when the serum antibodies are disappearing, induces a secondary response enhanced in time but not in antibody titers.