The immune system is a network of cells, tissues, and organs that work together to defend the body against attacks by “foreign” invaders. These invaders are primarily microbes — tiny organisms such as bacteria, parasites, and fungi that can cause infections. Viruses also cause infections, but are too primitive to be classified as living organisms. The human body provides an ideal environment for many microbes. It is the immune system's job to keep them out or, failing that, to seek out and destroy them.
When your body is attacked — perhaps by a virus or other germs — your immune system defends you. It "sees" and kills the germs that might hurt you. But when the system does not work right, this process can cause harm. Immune cells can mistake your body's own cells as invaders and attack them. This "friendly fire" can affect almost any part of the body. It can sometimes affect many parts of the body at once. This is called autoimmunity (meaning self-immunity).
When an intruder invades your body – like a cold virus or bacteria on a thorn that pricks your skin – your immune system protects you. It tries to identify, kill, and eliminate the invaders that might hurt you. But sometimes problems with your immune system cause it to mistake your body's own healthy cells as invaders and then repeatedly attack them. This is called an autoimmune disease. (Autoimmune means immunity against the self.)
You can think of the immune system as having two parts: the innate system and the acquired immune system.
-- the more primitive innate (or inborn) immune system activates white blood cells to destroy invaders. The innate system alerts the body to danger when it senses the presence of parts that are often found in many viruses or bacteria.
-- the acquired (or adaptive) immune system develops as a person grows. It “remembers” different invaders so that it can fight them better if they come back. When the immune system is working properly, foreign invaders (antigens) provoke the body to produce proteins called antibodies and specific types of white blood cells that help in defense. The antibodies attach to the invaders so that they can be recognized and destroyed.
The key to a healthy adult immune system is its remarkable ability to distinguish between the body's own cells, recognized as "self," and foreign cells, recognized as "nonself." In abnormal situations, the immune system can mistake self for nonself and launch an attack against the body's own cells or tissues. The result is called an autoimmune disease. Some forms of arthritis and diabetes (type 1) are autoimmune diseases. In other cases, the immune system responds to a seemingly harmless foreign substance such as ragweed pollen, and the result is allergy.
Autoimmune diseases refer to problems with the acquired immune system's reactions. In an autoimmune reaction, antibodies, or immune cells, attach to the body's own healthy tissues by mistake, signaling the body to attack them. Autoimmune disease is a disease that results when the immune system mistakenly attacks the body's own tissues.
It is well-known that good health is directly related to a strong immune system. Adults with weak immune systems can easily get sick. Eating a healthy diet, exercising regularly, controlling stress, and taking vaccines recommended by doctors are vital to having a strong adult immune system.
For many years, healthcare providers have used vaccination to help the body's immune system prepare for future attacks. Vaccines consist of killed or modified microbes, parts of microbes, or microbial DNA that trick the body into thinking an infection has occurred. A vaccinated person's immune system attacks the harmless vaccine and prepares for invasions against the kind of microbe the vaccine contained.