The immune system protects the body from infections, diseases and foreign substances. It involves a number of organs, tissues and cells. Whenever possible, these components work together to keep foreign invaders (known as antigens) from entering the body and bloodstream. However, when antigens do penetrate into the blood, the immune system is capable of immediately identifying and attacking the threat.

The immune system has a number of ways to combat antigens. The eyes, nose and mouth are all capable of keeping most types of antigens from penetrating the body through mucus and cilia (tiny hair-like projections).

Leukocytes (white blood cells) also play a very large role in the immune system. They identify and/or attack antigens that make it to the bloodstream. Some leukocytes aggressively bind themselves to antigens and release chemicals to kill them (such as killer T-cells). Others trigger the release of antibodies that are specifically designed to combat specific invaders.

The symptoms associated with an immune system response are often uncomfortable to the individual, but are actually quite necessary. Symptoms such as sneezing, runny nose, fever, coughing, itching, nausea, diarrhea or shortness of breath are all produced as the immune system tries to fight foreign invaders.

At times, the immune system mistakenly attacks a generally harmless substance. An allergic reaction is the most common example of such an attack. Other types of improper immune system reactions are responsible for type 1 diabetes, rheumatoid arthritis and other conditions. These are called autoimmune disorders.

The immune system, although functioning properly, may present problems for people undergoing organ transplants. The body perceives transplanted organs or body tissue as attacking foreign invaders and rejects them. This can have serious consequences for the donor recipient. Therefore, transplant patients are required to take medications to suppress this immune system response (immunosuppressants).