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The Immune System Definition
The immune system protects the body from infection. Allergies represent an inappropriate response from the immune system toward harmless substances. Doctors (MDs and DOs) who treat conditions of the immune system may be internists or immunologists. Doctors who specialize in treating allergies are allergists, and those who specialize in treating rheumatoid arthritis and related autoimmune disorders are rheumatologists.
Functions of the Immune System – How it Works
The immune system’s role is to protect the body from infection. Infection, from the immune system’s perspective, is any activity from foreign entities that causes damage to cells. It does so through a complex and intricate integration of organs, tissues, cells, and molecules.
Each day the bone marrow releases billions of monocytes and granulocytes, also called polymorphonuclear cells (PMNs), into the blood circulation. Monocytes circulate in the blood for about 24 hours and then migrate into the lymph tissues, liver, and the various mucosa-associated lymphoid tissue (MALT) structures throughout the body.
Known as macrophages after their migration, these cells participate in antigen processing as well as continued phagocytosis (consumption of cellular debris). Granulocytes (neutrophils, basophils, and eosinophils) are instrumental in the body’s inflammatory response, which is integral to healing in normal immune function as well as responsible for much of the distress of a hypersensitivity reaction—allergy—when the immune system malfunctions.
The workhorse cells of the immune system are the lymphocytes, which from birth divide into two camps: B-cell lymphocytes, which patrol the blood and lymph on the alert for invaders, and Tcell lymphocytes, which respond to the call of B-cell lymphocytes when invaders penetrate the body’s barriers. B-cell lymphocytes come to maturity in the bone marrow and regulate antibodymediated immunity. T-cell lymphocytes come to maturity in the thymus and regulate cell-mediated immunity. Lymphocytes circulate in the blood and the lymph, and also reside in lymph organs and tissues throughout the body. The spleen contains about half the body’s supply of lymphocytes.
Perhaps more than any other system of the body the immune system is one of molecular function. The entire function of the immune system centers on the ability of immune cells to distinguish cells that belong to the body—self cells—from cells that do not belong to the body—nonself cells. It does so through molecular markers called antigens. Lymphocytes, natural killer (NK) cells, macrophages, and the complement factors—a collection of substances that, when activated (the complement cascade), form potent chemical structures—key onto these antigens like lasers onto targets.
Cells bearing self antigens continue unimpeded about their business in the body. Those bearing nonself antigens are tagged for destruction by another set of molecular markers, antibodies, that specialized B-cell lymphocytes called plasma cells produce. Each individual antigen generates a different antibody; millions of antibodies circulate in the blood and lymph.
Filling out the immune system’s defense are specialized clusters of lymphoid tissue that line each point of access into the inner body: skin, nose, airways, gastrointestinal system, and even the blood vessels. These clusters—known collectively as the mucosa-associated lymphoid system—are like guard posts protecting the body’s vulnerabilities. MALT contains abundant populations of lymphocytes, mast cells, and macrophages that detect and intercept millions of microbes, viruses, toxins, and irritants before they can breach the inner body.
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