The Morphologic Responses of the Lymphoid System to Homografts. I. First and Second Set Responses in Normal Rabbits

The morphologic responses of the lymphoid system of the rabbit were studied. The results demonstrated: (a) the proliferation of a characteristic cell, the "hemocytoblast", within lymphoid centers; (b) architectural changes in lymph nodes and spleen, at first characterized by enlargement of follicles and germinal centers, later by effacement of normal landmarks and finally by reconstitution of lymphatic structures; (c) a barely perceptible plasmocytic response, especially in the first set reaction.

These changes, although most pronounced in the lymph node proximal to the homograft, were also found later in distant lymph nodes and in the spleen.

They arose more rapidly, were of greater intensity and were more persistent in the second set reaction than in the first set reaction. These findings indicated that transplantation immunity eventually invokes generalized morphologic changes in the lymphoid system. The events which followed a second set graft suggested that the second set phenomenon had the features of an anamnestic response.