In this paper we demonstrate several aspects of the mechanisms of action of the neurotransmitter Substance P in the immune system. We describe how Substance P can activate T cells, B cells, monocytes, and granulocytes to, respectively, proliferation, immunoglobulin synthesis, cytokine production, and chemotaxis. However, the neurotransmitter does not trigger cells of the immune system only via the well-characterized neurokinin-1 receptor, which mediates the signaling by Substance P in the neuroendocrine system. We show that Substance P can activate T cells receptor-independently. The receptor-independent activation of T cells leads to the activation of heterotrimeric G proteins and calcium-influx into the T cell, followed by an increase in proliferation of the cell. Apart from the receptor-independent activation pathway, Substance P can also activate monocytes and B cells via a nonneurokinin Substance P receptor. Activation of this novel receptor leads to the activation of MAP kinase, which is an important second messenger in the cascade leading to cytokine production by monocytes. In contrast to the non-neurokinin Substance P receptor, triggering of the NK-1 receptor, transfected in Jurkat cells, or triggering of T cells via receptor-independent pathways does not lead to activation of MAP kinase. Combining the data, we can conclude that the interaction between the neuroendocrine system and the immune system with regard to Substance P clearly indicates that the immune system does not necessarily mirror the communication pathways that are used in the neuroendocrine system. Substance P is capable of signaling the immune system via multiple activation pathways.