The immune system is a critical subsystem of the body, performing many different roles. For example, the immune system provides a physical barrier to foreign substances. Cells in skin are actually considered part of the immune system, and tears and nasal mucus contain natural chemicals that digest bacterial substances.

When you get an injury, your immune system reacts in two protective ways to prevent an infection from developing. The first response is based on inborn defenses collectively known as innate immunity, using a team of specialized immune cells. Some time later, a process called acquired immunity kicks in when your immune system "learns" about the problem and targets a specific response to fight an infection. This type of immunity involves the production of antibodies and certain specialized immune cells that are tailored against microbes or other foreign substances.

C-Reactive Protein Click for a larger view and caption

Many different types of cells fight on your immune system's front line. Each of these cell types performs a unique and necessary role in protecting you. In doing so, immune cells carry ammunition: chemicals and proteins that work to kill bacteria and disable viruses. Occasionally, immune cells overreact, causing this ammunition to inappropriately harm your own cells and tissues. Most of the time, this resolves fairly quickly on its own.

Another important arm of immune function is the complement system, which consists of a host of molecules that reside in your blood in an inactive form. Complement molecules spring to action when they sense bacteria, injury or other immune triggers. When activated, complement proteins cause a range of responses associated with starting and maintaining inflammation. These include causing changes in blood vessel walls (making them more permeable, or leaky), the summoning of various types of immune cells, as well as the production of other substances that promote inflammation. A molecule called C-reactive protein, or CRP, interacts with the complement system. The presence of CRP in blood is a telltale sign of inflammation because it is normally not present in appreciable amounts in the blood of healthy people.