The immune system in a vertebrate (an organism with a backbone) consists of all the cells and tissues that recognize and defend the body against foreign chemicals and organisms. For example, suppose that you receive a cut in your skin. Microorganisms living on your skin are then able to enter your body. They pass into the bloodstream and pass throughout your body. Some of these microorganisms are pathogenic, that is, they may cause illness and even death. As soon as those microorganisms enter your body, its immune system begins to identify them as foreign to your body and to produce defenses that will protect your body against any diseases they may cause.
The study of the immune system is known as immunology and scientists engaged in this field of research are immunologists. Our understanding of the way in which the immune system functions in animals has made possible the prevention of various diseases by means of immunizations. The term immunization refers to the protection of an individual animal against a disease by the introduction of killed or weakened disease-causing organisms into its bloodstream.
Words to Know
Antibody response: The specific immune response that utilizes B cells to kill certain kinds of antigens.
Antigen: Anything that causes an immune response in an animal.
B cell (or B lymphocyte): A lymphocyte that participates in the antibody response.
Helper T cell: A kind of T cell with many functions in the immune system, including the stimulation of the development of B cells.
Histamine: A chemical that causes blood vessels to dilate (become wider), thus increasing blood flow to an area.
Inflammatory response: A nonspecific immune response that causes the release of histamine into an area of injury; also prompts blood flow and immune cell activity at injured sites.
Lymphocyte: White blood cell.
Memory cell: The T and B cells that remain behind after a primary immune response and that respond swiftly to subsequent invasions by the same microorganism.
Nonspecific defenses: Immune responses that generally target all foreign cells.
Phagocytosis: The process by which one cell engulfs another cell.
Plasma cell: A B cell that secretes antibodies.
Proteins: Large molecules that are essential to the structure and functioning of all living cells.
Specific defenses: Immune responses that target specific antigens.
Vaccination: Introducing antigens into the body in order to make memory cells, thereby reducing the likelihood of contracting future diseases caused by those antigens.
Levels of defense
The immune system consists of three levels of response: external barriers; nonspecific responses; and specific responses. Included among the external barriers are the skin and mucous membranes. An animal's skin acts something like a protective wrapping that keeps diseasecausing organisms out of the body. Normally, the skin is covered with
One of the most exciting new disease-fighting agents is a class of compounds known as interferons. Interferons were first discovered in 1957 by Alick Isaacs and Jean Lindenmann. Isaacs and Lindenmann found that chick embryos injected with the influenza virus released very small amounts of a protein that destroyed the virus. The protein also prevented the growth of any other viruses in the embryos. Isaacs and Lindenmann suggested the name interferon for the protein because of its ability to interfere with viral growth.
Further research showed that interferon was produced within hours of a viral invasion and that most living things (including plants) make the protective protein. Scientists realized that interferons were the first line of defense against viral infection in a cell. They realized that interferons might be effective in treating a number of viral diseases in humans, such as some forms of cancer, genital warts, and multiple sclerosis.
Interferons are classified into two general categories, Type I and Type II. Type I interferons are made by every cell in the body, while Type II interferons are made only by T cells and natural killer (NK) cells. Interferons are also classified according to their molecular structure as alpha, beta, gamma, omega, and tau interferons.
In 1986, interferon-alpha became the first interferon to be approved by the U.S. Food and Drug Administration (FDA) for the treatment of disease, in this case, for hairy-cell leukemia. In 1988, this class of interferons was also approved for the treatment of genital warts, proving effective in nearly 70 percent of patients who do not respond to standard therapies. In that same year, it was approved for treatment of Kaposi's sarcoma, a form of cancer that appears frequently in patients suffering from AIDS.
In 1993, another class of interferon, interferon-gamma, received FDA approval for the treatment of one form of multiple sclerosis characterized by the intermittent appearance and disappearance of symptoms. Interferon-gamma may also have therapeutic value in the treatment of leishmaniasis, a parasitic infection that is prevalent in parts of Africa, North and South America, Europe, and Asia.
untold numbers of organisms, some that are harmless, but others that can cause disease. Virtually none of these organisms has the ability to penetrate the skin. Only when the skin has been broken, as in a cut, can the organisms pass into the body.
Mucous membranes are tissues that excrete a thick, sticky liquid known as mucus. All openings that lead to the interior of the body—the mouth, nose, anal tract, and digestive tract—are covered with mucous membranes. Organisms that try to enter the body through one of these openings tend to become trapped in the mucus, preventing them from entering the body.
Nonspecific immune system. Organisms that manage to penetrate the body's first line of defense then encounter another hurdle: the body's nonspecific immune system. The term nonspecific means that this line of defense goes into operation whenever any kind of foreign material enters the body. The immune systems of animals have developed the ability to tell the difference between its own cells, that is, cells produced by the body, and any other kind of material. The foreign matter might be another kind of organism, such as a bacterium or virus; cells from another animal; or inanimate matter, such as coal dust, pollen, cigarette smoke, or asbestosis fibers. Anything that causes an immune response in an animal is said to be an antigen.
Identification of foreign particles as "not-me" cells is made by a group of white blood cells known lymphocytes. Lymphocytes search out antigens in the bloodstream and destroy them by phagocytosis. Phagocytosis is the process by which one cell surrounds a second cell and engulfs it. Once the foreign cell has been swallowed up by the lymphocyte, it is digested by enzymes released from the lymphocyte.
The invasion of antigens can also produce an inflammatory response. Suppose you cut your finger on a tin can. The cut soon becomes red, swollen, and warm. These signs are evidence of the inflammatory response. Injured tissues send out signals to immune system cells, which quickly migrate to the injured area. These immune cells perform different functions. Some destroy bacteria by phagocytosis. Others release enzymes that kill the bacteria. Still other cells release a substance called histamine. Histamine causes blood vessels to dilate (become wider), thus increasing blood flow to the area. All of these activities promote healing in the injured tissue.
Allergic reactions are examples of an inappropriate inflammatory response. When a person is allergic to pollen, the body's immune system is reacting to pollen (a harmless substance) as if it were a bacterium and an immune response is
prompted. When pollen is inhaled, it stimulates an inflammatory response in the nasal cavity and sinuses. Histamine is released, which dilates blood vessels and causes large amounts of mucous to be produced, leading to a "runny nose." In addition, histamine stimulates the release of tears and is responsible for the watery eyes and nasal congestion typical of allergies.
To combat these reactions, many people take drugs that deactivate histamine. These drugs, called antihistamines, are available over the counter and by prescription. Some allergic reactions result in the production of large amounts of histamine, which impairs breathing and necessitates prompt emergency care. People prone to these extreme allergic reactions must carry a special syringe with epinephrine (adrenalin), a drug that quickly counteracts this severe respiratory reaction.
Specific immune system. The body's third line of defense against invasion by foreign organisms is the specific immune system. The specific immune system consists of two kinds of lymphocytes known as T lymphocytes and B lymphocytes. The two kinds of cells are sometimes known simply as T cells and B cells. Both kinds of cells are produced in bone marrow. T cells then migrate to the thymus (which gives them the T in their names), where they mature. No one knows where B cells mature.
T cells and B cells differ from nonspecific lymphocytes in that they attack only very specific antigens. For example, the blood and lymph of humans have T cell lymphocytes that specifically target the chicken pox virus, T cell lymphocytes that target the diphtheria virus, and so on. When T cell lymphocytes specific for the chicken pox virus encounters a body cell infected with this virus, the T cell multiplies rapidly and destroys the invading virus.
Two kinds of T cells exist: killer T cells and helper T cells. Killer T cells go directly to the target antigen and attack it. Helper T cells have many different functions, including to help in the development of B cells. Another function is to stimulate the formation of other T cells and the release of various chemicals that aid in the destruction of antigens.
Helper T cells have an especially crucial role in the immune system. Thus, any disease that destroys helper T cells has a devastating effect on the immune system as a whole. HIV (human immunodeficiency virus, which causes AIDS [acquired immunodeficiency syndrome]), for example, infects and kills helper T cells, thus disabling the immune system and leaving the body helpless to stave off infection.
Memory cells. After an invader has been destroyed, some T cells remain behind. These cells are called memory cells. Memory cells give an animal immunity to future attacks by the original invader. Once a person has had chicken pox, memory cells are created. If the person is later exposed to the chicken pox virus again, the virus is quickly destroyed. This secondary immune response, involving memory cells, is much faster than the primary immune response.
The procedure known as vaccination makes use of the above process. Vaccination is the process by which a killed microorganism (or parts thereof) are injected into a person's bloodstream. The presence of these particles prompts the formation of memory cells without a person's having to actually develop the disease.
B cells and the antibody response. When helper T cells recognize the presence of an invading antigen, they stimulate B cells in the blood and lymph to start reproducing. As the B cells reproduce, they also undergo a change in structure and become known as plasma cells. Those plasma cells then begin to secrete compounds known as antibodies. Antibodies are chemicals released by B cells that attach themselves to the surface of an antigen. The presence of an antibody helps other cells in the immune system recognize the antigen and mark it for destruction.