Cytokine-producing cells of the immune system are diagrammatically illustrated together with lists of cells activated or influenced by the secreted cytokines. As a prelude to mounting an attack against foreign compounds, cells of the immune system must discriminate between normal endogenous compounds. This chapter presents certain functional elements will be discussed to provide a foundation for understanding the neuroendocrine immunology. Activation of the cytokine network, as occurs when the immune system detects non-self antigens or there is tissue injury, initiates a cascade of cellular and hormonal events, some of which involve the neuroendocrine system. The neuroendocrine axis that is most affected by blood-borne cytokines is the hypothalamus-hypophysis-adrenocortical axis. Endocrine and neuroendocrine cells are influenced by both blood-borne and locally produced cytokines. Cytokines produced by activated lymphoid cells and macrophages may act locally on adjacent cells or travel in the systemic blood to distant sites for action.