LYMPHOID-TISSUE STRUCTURE AND LYMPHOCYTE TRAFFICKING IN THE PIG

The organised lymphoid tissues of the pig, though conventionally mamma lian in most respects, show several distinctive properties in their st ructure and physiology. Specialised antigen-presenting lymphoid organs function at three compartmental levels: the body surfaces, their drai ning lymph nodes and the spleen in the bloodstream. Other organs act a s lymphocytic depots and sites of phagocytosis of debris. Pig lymphocy tes recirculate continuously through these organs and through differen t forms of inflammatory change, experimentally induced by mitogens and cytokines, using a spectrum of distinctively different physiological mechanisms. These tissues in the young pig differ both in the resting and activated cell subsets and the molecules involved, many of which r emain to be completely defined. Intriguing insights are evident in the subtlety of regulation of the specificity, level and foetal ontogeny of trafficking mechanisms in these different tissues, though they are as yet poorly explained. This subtle molecular physiology is only now emerging because appropriate monoclonal antibody reagents are being de veloped and rigorous attention is being paid to the use of gentle phys iological methods in experiments in vivo.