Reassortment of cell populations within the lymphoid apparatus of the sheep.

As lymphocytes recirculate through the blood tissues and lymph they are sorted into populations which have varying morphological and functional characteristics. Lymphocytes are added, deleted and transformed within the lymphoid apparatus as a consequence of non-random migration and antigenic stimulation. There is evidence that the physiological characteristics of peripheral and central lymph nodes vary as a result of differences in the origins of the cells entering the nodes. Lymphocytes enter the lymph nodes from the blood and lymph in varying numbers; consequently the cell population in the efferent lymph of central and peripheral lymyph nodes contains different proportions of blood-borne and lymph-borne cells. Cells arriving in lymph nodes by way of the blood or the lymph migrate differently within the node. Those entering from the blood go principally to the paracortex and the follicular areas. Lymphocytes entering in the lymph are distributed through both the cortex and the medulla. In humoral antibody responses and in the response that occurs during the rejection of a renal allograft, lymph-borne cells populate the medullary cords, cortex and germinal centres of the nodes they enter. Within these nodes, new populations of cells are generated which have different functional attributes from the cells which provoked their formation.