1. Studies of the immune response have been carried out in more than 1700 lampreys representing three stages in the life cycle of these animals.

2. Lampreys used in this study were unable to clear certain soluble protein antigens and bacteriophage and were unable to make antibodies to these antigens. Hemocyanin was cleared from the circulation.

3. The immune responses demonstrated in lampreys include the production of specific antibody to killed Brucella cells, the rejection of skin homografts, and the development of a delayed allergic response to old tuberculin.

4. A responsive proliferation of lymphoid cells occurred in the protovertebral arch following antigen-adjuvant stimulation.

5. Electrophoretic and immunoelectrophoretic analysis of lamprey serum revealed gamma globulin. Ultracentrifugal analysis of serum revealed proteins with sedimentation coefficients of 17S, 8S, 7S, and 3S.

6. The antibodies thus far observed in the lamprey are of relatively high molecular weight and destroyed by 2-mercaptoethanol.

7. In the lamprey it would appear that there is reflected the coordinate evolution of a primitive thymus, primitive spleen containing lymphoid foci, a family of lymphocytes in the peripheral blood and capacity for gamma globulin synthesis and expression of adaptive immunity.

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