The immune system has a complex and dynamic relationship with the nervous system, both in health and disease. The immune system surveys the central and peripheral nervous systems, becoming activated in response to foreign substances, infectious particles or neoplasms. Conversely, the nervous system modulates immune system function both through the neuroendocrine axis and through vagus nerve efferents. In disease states, this dynamic relationship is perturbed, resulting in neuropsychiatric diseases. In this manuscript, we will summarize fundamental principles of the immune system and its interaction with the nervous system. We will describe the critical components of the adaptive and innate branches of the immune system and will describe important effectors and signalling pathways in each. By understanding the principles of the immune system and how these principles relate to nervous system function, the reader will be prepared to interpret subsequent manuscripts in this issue.