The immune system is different from other systems we have considered up to this point. Most organ systems are made up of discrete tissues or organs. Consider the cardiovascular system, which includes the heart and associated blood vessels. Or think of the gastrointestinal tract, which in vertebrates consists of a series of connected tissues and spaces, each with a specialized function for digesting and absorbing nutrients from food.

By contrast, the immune system consists of organs, cells, proteins, and even biochemical reactions that are spread throughout the body. This makes sense, in that the immune system must be able to interact with pathogens wherever they might be found. There are some organs that are dedicated to the immune system, such as the thymus in many vertebrates (next page), but otherwise it’s a diffuse system of cells, molecules, and processes. In fact, the phrase “immune system” was not even used until sometime in the late 1960s. It was introduced to draw attention to the fact that it’s truly a system made up of many different components. These components work by different mechanisms, but all share a common function: to protect an organism from pathogens.