Overview of the Immune System
Our bodies have several layers of defense against pathogens and other invaders from the environment. Although it may not be the first thing to come to mind, the skin is one of the primary protectors of our bodies, offering a physical barrier. The skin naturally secretes oils that have antibacterial properties. Yeast and friendly bacteria also live in the skin, occupying the space so that harmful bacteria will not be able to move in and take over.
The mucus in the mouth, nose, and lungs also helps defend the body by trapping pathogens and other particles so that they cannot continue deeper into the body. Any such moisture surrounding an entrance into the body plays this very important protective role. Tears and saliva, in addition to being able to flush potential invaders out, also contain antibacterial enzymes to combat infection.
Within the lungs, particles can get caught in the thicker mucus produced in the lungs known as phlegm. Through the motion of cilia, small hair-like protrusions in the lungs, the phlegm slowly makes its way upward out of the lungs to be released nasally or orally.
The extreme acidity of the stomach will kill most ingested pathogens, protecting the digestive tract from infection. Just as our skin is infested with microorganisms, so are our digestive tracts. The body keeps a healthy population of friendly bacteria throughout the gastrointestinal tract to prevent the colonization of harmful bacteria.
Should any of these layers be breached, the invading pathogens will be dealt with by the internal collection of biological processes associated with the immune system. Broadly, these immune defenses can be grouped into two categories. First, there is the innate immune response, which is a general protection system that always responds the same way no matter what the specific attacker is. The body does not recognize anything specific about the attacker; it just knows that it should be removed and quickly responds to it. Afterward, the body's immune system will have no memory of the pathogen it eliminated, so in encountering it again, it will respond in the same way.
In the case of an adaptive immune response, however, the immune system will retain a memory of the pathogen that it fought. In this type of immune response, the body generates antibodies to the specific pathogen, in essence, preserving a record of the infection. This sort of immune response, because it is more specific, is less immediate. There is some lag between exposure to the pathogen, generation of the antibodies, and the immune response. If the body encounters the infection once again in the future, however, it will remember it and can deal with it very quickly and effectively. This is the principle behind vaccination. Viral vaccines, for example, are usually attenuated (harmless) versions of the viruses they are meant to protect against. By exposing the body to this version of the virus early in life, the body will generate antibodies and an immunological memory of the virus, so that if it ever encounters it again, the immune system can rapidly dispose of it.
Types of Immune System Disorders
Immune system disorders fall into three categories: immunodeficiency, autoimmunity, and hypersensitivity.
Immunodeficiency occurs when some part of the immune system is unable to fight off infections. There are rare genetic diseases, for example, where the cells that the immune system normally sends to fight the pathogen are defective and unable to find (or fight) the invader. More often, immunodeficiency is acquired and there are several ways this can occur. Aging, for example, can reduce a person's immunity, as can malnutrition. Certain medications, such as those used in chemotherapy, can also significantly suppress the immune system and lead to immunodeficiency. Several types of cancer can also lead to immunodeficiency, such as lymphoma, multiple myeloma, and leukemia. AIDS (acquired immunodeficiency syndrome) is characterized the by the loss of immunity caused by the HIV (human immunodeficiency virus). The infections that occur in AIDS patients due to the extreme suppression of the immune system are what make it so deadly.
In the case of adaptive immunity, the body is normally very good at attacking pathogens while recognizing normal physiological proteins as safe. If the body fails to make this distinction and attacks its own proteins, this is known as autoimmunity. Because the immune system cannot eradicate our own proteins, it puts the body in a constant state of attack and defense, inducing chronic inflammation and other tactics normally reserved for pathogens. Rheumatoid arthritis, type I diabetes, and lupus are examples of autoimmune disorders.
Occasionally, the body's immune system overreacts to a mild challenge, potentially damaging its own tissues. Allergies are an example of such a hypersensitivity disorder. (Please see our article on allergies for more information.) When the body encounters the allergen, it mounts an overzealous immune response which leads to inflammation that may be limited to the area of contact or spread throughout the entire body. Some autoimmune disorders are also hypersensitive reactions to self proteins, such as multiple sclerosis.
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