Antigen in tissues. 3. The separation of antigen-containing components from lymphoid tissues.

Particulate and soluble antigens were labelled with I, injected into rats and the lymphoid organs examined. As determined by autoradiography of tissue sections, one antigen used localized exclusively in vacuoles of medullary macrophages of the lymph nodes and others on the surfaces of reticular cells in the lymphoid follicles of nodes or the white pulp of spleen. The remaining antigens studied localized in both medulla and lymphoid follicles of nodes. Tissues containing antigen were homogenized in a sucrose medium and most radioactivity was recovered in a large granule fraction. This fraction was submitted to equilibrium centrifugation. The preparations were not resolved in gradients of sucrose or dextran but in gradients of Urografin the preparations were resolved into two or more peaks of radioactivity. Medullary localized antigen banded in a region of the gradient rich in lysosomal enzymes and was considered to be present in vesicles. Antigen was not found in a region of the gradient rich in mitochondria. Antigen from lymphoid follicles of nodes or from spleen white pulp banded at high density values and was considered to be present as an antigen—antibody complex, possibly associated with membrane. Equilibrium density centrifugation in Urografin gradients provides a means of separating and examining the properties of antigen in lymphoid tissues.