Rat alloantisera of the following specificities were obtained after single skin graft rejection: DA anti-BN, BN anti-DA, DA anti-Lewis, and Lewis anti-DA. Anti-alloantibodies were raised by injecting the first two alloantisera into (DA x BN)F1 hosts and the last two alloantisera into (DA x Lewis)F1 hosts. In this manner, four anti-alloantisera were raised: anti-(DA anti-BN), anti-(BN anti-DA), anti-(DA anti-Lewis), and anti-(Lewis anti-DA). From each anti-alloantiserum a gamma globulin fraction was prepared and trace labeled with 125I.

The labeled anti-alloantibodies could be shown to be fixed preferentially by red cells coated with the alloantibody used to induce them. They were also preferentially fixed by normal rat lymphoid cells presumed to carry recognition structures similar to alloantibody. For instance, anti-(DA anti-BN) was fixed by BN red cells coated with DA anti-BN and by normal DA lymphoid cells presumed to carry RS(BN). Conversely, anti-(BN anti-DA) was fixed by DA red cells coated with BN anti-DA and by normal BN lymphoid cells, carrying RS(DA).

Absorption and inhibition experiments showed that alloantibodies and normal lymphoid cells compete for the same labeled anti-alloantibodies. We conclude that cellular receptors and circulating alloantibody are antigenically similar: They have the same aliotype.

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