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The immune system is a complex collection of specialized organs, tissues and cells spread throughout the human body that protects the body from outside cells or threats. This allows a rapid response by almost every part of the body. With a variety of body components acting together, the immune system is capable of quickly overcoming most foreign invaders.

The immune system’s flexibility is a result of its complexity. Most organ systems have defined parts and functions within the body, such as the heart and blood vessels of the circulatory system or the stomach, intestines and other organs that make up the digestive system. In addition to their primary role in the body, many of these systems and bodily tissues also help to fight foreign invaders. The immune system must be capable of fighting threats from any source outside the body and even from cells inside the body.

The body is regularly threatened by pathogens (viruses, bacteria, fungi, parasites) and toxins that can be found in the environment. When these materials penetrate the body’s barriers (e.g., skin, mucous membranes), the immune system works to prevent or limit the damage they can cause. Disease occurs when the immune system does not adequately function.

Before foreign invaders can cause infection or tissue damage, they must pass through barriers. Some body systems provide these barriers, in addition to their main function in the body. These body systems include:

• Skin. The body’s largest organ blocks toxic substances and pathogens in the environment from reaching internal tissues and organs.
  
  
• Mucous membranes. The moist lining of body cavities and passageways, including the inside of the mouth, nose and eyes, form a barrier to trap foreign substances and organisms. The upper respiratory tract is also protected by internal cilia (tiny hair-like structures) that further help to trap these antigens. The foreign invaders are then removed from the body. Materials caught in the mucous membranes of the mouth and nose may be expelled through coughs and sneezes.
  
  
• Digestive system. Pathogens may be consumed with food. They are usually killed in the stomach by acids that are part of the digestive process.

If foreign invaders get past these barriers, the main organs and cells of the immune system may become engaged.

The immune system produces many reactions throughout the body to help it fight infection. In general, it requires inflammation of tissues to be effective, which causes redness, heat and swelling. The redness results from the dilation, or widening, of blood vessels in the area of infection. This allows more white blood cells, which are highly involved in fighting disease, to reach the area.

Inflammation also raises the temperature of the infected area or much of the body. This helps chemicals the body makes to kill foreign organisms to be more effective. It also allows the cells that fight infection to reproduce more rapidly. The resulting fever may be uncomfortable for the patient, but is important for the immune system to work.

Swelling related to inflammation is often associated with the collection of cells and fluids in the infected area. This may include immune system cells, bacteria that have been killed and chemicals used in the body’s response. Gradually they are removed from the tissues by the circulatory and lymphatic system.

Other signs that the immune system is functioning include sneezing, coughing and a runny nose. These are all mechanisms used to eliminate pathogens and foreign materials from the body.