Local immune responses in afferent and efferent lymph.

Abstract

Acquired immune responses are dynamic processes dependent upon the co-ordinated interaction of antigen-trapping accessory cells and lymphocytes of the immune system in specialized secondary lymphoid tissues that include lymph nodes, Peyer’s patches and spleen. The function of the immune system is to clear infections or neoplastic changes locally wherever they appear while avoiding autoimmune responses and non-specific damage of unaffected tissues. Mature lymphocytes with a broad range of potential antigen specificities that are not autoimmune are generated in the primary lymphoid tissues. Many, if not all, of these circulate between the blood and the tissues by way of the lymph nodes. The majority of small lymphocytes recirculate between the blood and secondary lymphoid tissue either directly through post-capillary venules or indirectly via tissues such as the skin and the gut mucosa.1–5 Blood CD4+ and CD8+ T cells recirculate efficiently, whereas a subset of B cells and γδ T cells do not.6–8 T and B cells (particularly CD4+ T cells) from peripheral lymph nodes and from gut-associated lymphoid tissue tend to recirculate to the tissue of origin,3–5,9 although this tissue-specific recirculation is by no means absolute. Recirculating lymphocytes can, therefore, monitor tissues for the presence of foreign antigens and enhance the recruitment of antigen-specific cells to the local lymph node. This tissue-specific immune surveillance by lymphocytes is facilitated by a network of lymph nodes distributed strategically around the body and connected by a system of lymphatic vessels. The purpose of this article is to review recent advances in our understanding of the dynamics of local immune cell and cytokine responses in the two strikingly different compartments of afferent and efferent lymph that, respectively, enter and exit peripheral lymph nodes. The emphasis will be on studies in afferent and efferent lymph entering and exiting skin-draining lymph nodes in ruminants where lymphatic cannulation is relatively easy to perform.