Thymic dependence of amphibian antibody responses

Larvae of Xenopus laevis were thymectomized at one week post fertilization and tested as toadlets for their ability to produce circulating antibodies. Sham‐thymectomized animals immunized with human IgG in adjuvant produced both high (IgM) and low molecular weight antibody, while in those immunized with sheep red blood cells, antibody, was detectable only in the IgM moiety. Thymectomized siblings, though apparently healthy, failed to respond to either antigen. The inhibition of IgM antibody in the amphibian following thymic depletion contrasts with the mammal, where normal or subnormal titers can still be demonstrated. This may point to phylogenetic differences in the role of the thymus. Alternatively, early‐thymectomized Xenopus may simply be closer to a completely athymic condition.