ABSTRACT
The immune system is composed of many cell types, the majority of which are organized into separate lymphoid tissues and organs (Section C2). Because attack from microbes can come at many different sites of the body, the immune system has a mobile force of cells in the bloodstream that are ready to attack the invading microbe wherever it enters the body. Although many of the cells of the immune system are separate from each other, they maintain communication through cell contact and molecules secreted by them (such as cytokines and chemokines). Like the other body systems, the immune system is only apparent when it goes wrong. This can lead to severe, sometimes overwhelming, infections and even death. One form of dysfunction is immunodeficiency, which can, for example, result from infection with the human immunodeficiency virus (HIV) causing acquired immune deficiency syndrome (AIDS). On the other hand, the immune system can be “hypersensitive” to a microbe (or even to “an inert” substance such as pollen) and this itself can cause severe tissue damage sometimes leading to death. Thus, the immune system must strike a balance between producing a life-saving response and a response that causes severe tissue damage. This regulation (Section G) is maintained by cells and molecules of the immune system (e.g., regulatory T cells-Tregs, and cytokines) and from without by nonimmune cells and tissues and their products (e.g., the neuroendocrine system).
