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Immune System
The invisible battle goes on day and night. There are many enemies lurking about, waiting to attack. On guard is a tireless army of defenders, posed and ready to fight. These defenders are part of the immune system—a remarkable system that protects the body from harmful invasions by foreign substances and disease-causing organisms.
Immunity
The body's ability to protect and defend itself against a foreign enemy agent is called immunity. There are several types of immunity. Natural immunity is a type of general inherited protection. A person is born with natural immunity, and it is species specific—for example, people do not get cat distemper and cats do not get human colds. Active immunity is the protection a person develops during his or her lifetime as the result of actually having been immunized against a disease. One way this can be done is by using vaccines to help an individual develop active immunity. Active immunity is long-lived and may even last a person's lifetime. Passive immunity is a temporary form of immunity that is borrowed from another source. It is short-lived, only lasting a matter of weeks. A baby receives passive immunity from its mother before it is born and after it is born from the mother's milk. Passive immunity protects the baby until it is old enough to have its "baby shots." Then the baby will build its own active immunity.
When the immune system sets up an attack against a foreign agent, it must first recognize which cells belong to the body and which do not so that it can selectively destroy the invader while protecting the healthy body cells. The immune system can tell the difference between what is self (the body's cells and tissue) from something that is foreign because of the chemical labels that appear on the outside surface of all body cells. The cells in a person's body have chemical self labels; each person has their own unique set of self labels. Invading agents also have chemical labels; each invading agent has foreign antigen labels.
Each foreign antigen has its own chemical configuration—just as each person has a unique set of human fingerprints. Because each foreign antigen has its own unique set of chemical fingerprints, the immune system is able to recognize the difference between foreign agents and develop a separate defense against each.
The Immune Response
The immune response occurs in stages. When an organism invades the body, it must be recognized as foreign. Once it is recognized, it must be marked so that all the cells within the immune system will know that it is an invader and is attacking the body. Then the various troops of immune cells must be called to battle, a defense plan organized, and the defense begun. Finally, when the enemy has been defeated, a cease-fire must be sounded and the battle stopped.
Immune Defenses
The immune system has two defense plans: nonspecific and specific. The nonspecific defense is used against all invaders. Nonspecific responses are only able to recognize the difference between self cells and foreign antigens. The response to the foreign antigens occurs quickly and does not require having had previous encounters with the outside invaders.
In a specific defense, the immune system recognizes the invader as one having previously attacked the body. Specific immune responses not only recognize self cells as different from foreign antigens; but they recognize the difference between the different kinds of foreign antigens.
Immune Cells and Molecules
The cells of the immune system fall into three groups: macrophages, natural killer cells, and lymphocytes. All types of cells are members of the white blood cell population and have their origin in bone marrow. These defender white cells account for one percent of the body's 100 trillion cells.
Macrophages.
The macrophages are the first to the battleground and are part of a nonspecific defense. They swallow up and dispose of enemy agents, dead cells, and other debris. They are phagocytes, or "cell eaters." Not only do they ingest foreign substances, but macrophages also process harmful material so that it can be recognized as dangerous by other immune cells. These phagocytic cells constantly wander about, patrolling all the areas of the body. They are found in the bloodstream, tissues, and lymphatic system.
Natural Killer Cells.
Natural killer cells are a group of special immune cells that are able to quickly identify and destroy a large variety of newly formed cancer cells, virus-infected cells, and fungi. These immune cells are nonspecific in their attack.
Lymphocytes.
The lymphocytes follow the macrophages into battle against foreign invaders. Unlike the nonspecific macrophages, lymphocytes are part of a specific immune defense—they are programmed to recognize and destroy specific enemy agents. For example, one group of lymphocyte defenders is trained to recognize and protect against measles and another to protect against chicken pox.
There are two kinds of lymphocytes: T lymphocytes (T cells) and B lymphocytes (B cells). Although both types of lymphocytes are formed in the bone marrow, the T cells leave the bone marrow before they are fully developed and travel to the thymus gland. In the thymus gland, they mature and are programmed in the skills needed to function as a T cell. The B cells remain in the bone marrow to receive their B cell programming.
B Cells.
When confronted with a specific enemy agent, some B cells are programmed to produce immune molecules called antibodies that recognize the chemical fingerprints of the foreign antigen. Each antibody is made up of long chains of smaller units in a pattern. The smaller units are amino acids, and it is their pattern that matches the chemical fingerprints of a specific foreign antigen—much like a lock and key match each other.
Antibodies do not attack the enemy directly. In battle with foreign invaders, some antibodies may call phagocytic cells to the battleground, mark the enemy so that it can be recognized and destroyed, or empty out the enemy cell by punching holes in its surface.
When the enemy agent has been defeated, other B cells store information about the invader for the future. If the foreign agent invades the body again, these memory B cells quickly recognize the foreign antigen and signal the production of antibodies to begin.
T Cells.
There are four kinds of T cells: T helper cells, cytotoxic T cells, suppressor T cells, and memory T cells. The immune response begins with the command from the T helper cells. They recognize the invader, then call in and train other immune cells, including B cells and cytotoxic T cells. The cytotoxic (cell killing) T cells kill the enemy directly. T suppressor cells decide when the battle ends—they call a truce to the fighting and shut off the immune system. Memory T cells remain, ready to move quickly should the enemy invade again.
Organ Transplants and the Immune System
Sometimes illness or an accident damages body tissue so much that it is no longer able to function normally. Then an operation, called a tissue graft, or transplant, may be performed to replace the damaged part. Healthy tissue, such as skin, may be taken from one part of the body and grafted to another part of the body. The body accepts the transplanted tissue because it has the same self label. Problems arise when the tissue or organs are taken from one person and transplanted to another. Then the immune system recognizes the transplant as foreign and launches as attack against it. When this happens, special drugs are used to lessen the immune system attack.
Although the body attacks cells that have a different self label, some self labels are chemically more similar than others and cause less of an immune response. The self labels are coded by genes inherited from each parent, so brothers and sisters are more likely to have similar, but not the same, self labels. The only people who would have the very same self labels would be identical twins, because they share the same genes.
Disorders of the Immune System
Sometimes the immune system works too hard and an allergic response results or it mistakes the body's own tissue for an invader and an autoimmune (self-immune) disease results. Other disorders result when cells in the immune system are attacked and destroyed by an organism, such as the AIDS virus, which destroys the T helper cell.
Allergies
Allergies are the result of an overactive immune system. In some people, the immune system mounts a large-scale attack against common environmental agents that should pose no threat to the body. Then unnecessary antibodies are produced against such things as house dust, animal dander, tree and weed pollens, and in some cases the chemical molecules in certain foods.
Autoimmune Disease
Autoimmune disease is a disorder of mistaken identity. It occurs when the body has difficulty distinguishing between self and foreign enemies. In rheumatoid arthritis, for example, the body's immune system acts as though parts of the knee, hand, or foot joints were foreign enemies, and it attacks them. The reason that some people develop an autoimmune disease and others do not is unknown. It is thought that some chemical agent or infectious organism may alter the self label so that the immune system recognizes certain tissues as enemies.
AIDS
Acquired immune deficiency syndrome (AIDS) is a disease that destroys the immune system. AIDS is caused by the human immuno-deficiency virus (HIV). Like all other viruses, HIV must seek out living cells to infect and in which to reproduce; it is not able to live on its own. The AIDS virus seeks out the cells of the immune system, particularly T helper cells and macrophages, to infect. Eventually the AIDS virus destroys the cell. Without the T helper cell, the immune system is unable to organize a defense against the invaders. Thus what are normally harmless organisms can cause life-threatening infections in people who have AIDS.
Immune Discoveries
Immunology, the study of the immune system, started long ago—even before people knew there was such a thing as the immune system! By the 1400's there was a primitive form of immunization. A powder made from dried smallpox scabs was inhaled as protection against getting smallpox.
Modern immunology really began with the work of two immunologists, the scientists who study the immune system. It was in 1796 that the first modern vaccination was developed by Edward Jenner, a physician in England. He noticed that herdsmen and dairymaids who had been infected with cowpox, a mild viral disease affecting cows, did not get smallpox. Smallpox was a much more serious and often fatal disease. He used this information to develop a method of infecting people with cowpox to protect them from smallpox. The term vaccination (vacca meaning "cow") was used to describe Jenner's method.
About one hundred years later, the French scientist Louis Pasteur discovered that chickens infected with a mild form of chicken cholera did not develop the more serious form of chicken cholera. To infect the chickens, Pasteur developed a weakened form of the disease-producing organism.
Since Pasteur's time, vaccines have been developed for many other diseases, including polio, measles, mumps, tetanus, whooping cough, and influenza. And methods have been developed to classify different types of blood and tissues, diagnose cancer, and examine genes and their effect on cell function.
New Weapons for the Future
Vaccines are now used to fight cancer. For example, there is a vaccine that acts against the virus that causes cervical cancer. Some vaccines act against cancer cells, such as those in prostate cancer. Another way to fight cancer is to use monoclonal antibodies. These are laboratory-grown cells that produce huge amounts of specific antibodies.
In 1988, the first HIV vaccine trial began. In 1999, the HIV Vaccine Trials Network (HVTN) was formed. In the HVTN, sites around the world test new HIV vaccines.
And studies are being carried out to assess factors that may affect the immune system. These factors include diet, exercise, stress, and exposure to pollution.
Carol Mattson Porth
Author
Pathophysiology: Concepts of Altered Health States
Author
Pathophysiology: Concepts of Altered Health States
How to cite this article:
- MLA (Modern Language Association) style:
- Porth, Carol Mattson. "Immune System." The New Book of Knowledge. Grolier Online, 2011. Web. 5 Oct. 2011.
- Chicago Manual of Style:
- Porth, Carol Mattson. "Immune System." The New Book of Knowledge. Grolier Online. 2011. http://nbk.grolier.com/article?id=a2014390-h&product_id=nbk (accessed October 5, 2011).
- APA (American Psychological Association) style:
- Porth, C. M. (2011). Immune System. In The New Book of Knowledge. Retrieved October 5, 2011, from Grolier Online http://nbk.grolier.com/article?id=a2014390-h&product_id=nbk
