Interrelation of the avian immune and neuroendocrine systems

Our understanding of immunology and endocrinology has been enriched by studies with different avian species. Numerous experiments have identified the role of the avian thymus and bursa of Fabricius in the immune response and the interrelation of these glands and their cellular products with the neural endocrine system. The growth pattern of the thymus and bursa are influenced by steroids. Increases in thymi growth appear to be controlled, in part, by fluctuations in thyroid activity. Sex-linked dwarf (SLD) chicks have lower T3 levels than normal chicks, but normal levels of growth hormone. The poor agglutinin response of SLD chickens to low concentrations of sheep red blood cells (0.1-1.0%) may be overcome by increasing the percentage of sheep erythrocytes to 5 or injecting growth hormone, which may enhance the conversion of T4 to T3. Castration impaired cell-mediated immunity but surgical removal of the thyroid did not significantly influence the humoral response to sheep erythrocytes or Brucella. Embryonic hypophysectomy reduced the number of B cells in the late embryo. Lesioning the anterior pituitary in 9-day-old chicks markedly reduced several parameters of cell-mediated immunity. In ovo bursectomy at 68 hr markedly increased the size of the adrenals and testes, numbers of Leydig cells, and levels of circulating testosterone but not corticosterone. On the basis of identification of lymphokines or cytoplasmic granules, lymphocytes and other cells of the lymphomyeloid complex are candidates for an endocrine role. A circuitry between the lymphocyte and neuroendocrine system has been identified in mammals and should be extended to Aves.