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The Immune System: A Primer

1995

A note from TheBody.com: Since this article was written, the HIV pandemic has changed, as has our understanding of HIV/AIDS and its treatment. As a result, parts of this article may be outdated. Please keep this in mind, and be sure to visit other parts of our site for more recent information!

This excerpt is reprinted by permission of Penguin Books USA, Inc.
from The Immune Power Personality by Henry Dreher

As a guide to the research detailed in this book, I offer a primer on the basic functions of our immune system--its organs, cells, cell products, and messenger molecules. This system is a fluid network designed to protect us from agents of disease, and to heal wounds delivered by injury or invasion. One immunologist called it a "roving bag of cells" that patrols our bodies on missions of resistance and restoration.

In order to do its job properly, our immune system must be exquisitely sensitive in detecting the surface features of other cells and substances. It must distinguish the "fingerprints" of intruders from those of family members--our own cells and molecules. That is why scientist Ted Melnechuk once remarked that "The immune system is a sensory organ for molecular touch." But our defense network is called upon to be as aggressive as it must be sensitive. Its task is to indentify and then eliminate foreign agents--be they bacteria, viruses, fungi, toxic chemicals, or cancer cells--with precision and dispatch.

Except for the nervous system, the immune system is the most complex biological system we have. It consists of master glands, principally the thymus; various sites that harbor immune cells; and different classes of "soldier" cells, which carry out specialized functions--including cells that prompt, cells that alert, cells that facilitate, cells that activate, cells that surround, cells that kill, even cells that clean up. Many immune cells also synthesize and secrete special molecules that act as messengers, regulators, or helpers in the process of defending against invaders.

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Antigens: The Signalers. Antigens are the fingerprints of immunity. They are identifying molecules that reside on the surface of cells and, like fingerprints, are unique to the cells that bear them. All of our body cells have antigens that signal "self-self-self"-- a message that they are part of us and therefore not to be attacked.

Microorganisms, viruses, or any agent that invades our bodies also have identifying antigens on their surfaces, which signal "foreign-foreign-foreign" to the immune system, readying it for immediate attack. That's why organ transplants are difficult; the antigens on newly-introduced cells sound the "foreign" alarm. To prevent the rejection of transplanted tissues, a patient is given drugs that suppress the immune system.

If the immune system overreacts to an outside antigen, the result is an allergy. Hayfever, for example, is a hyperresponse to grass, pollen, or ragweed antigens. When our immune system reacts inappropriately to the antigens on our own cells, the result is an autoimmune disorder. Lupus erythematosus and rheumatoid arthritis are examples of autoimmune diseases, in which our own tissues are atttacked from within by our immune defenses.

If our immune systems fail to react properly to an outside agent--say a virus or bacterium--the result is an infection. Finally, if our immune systems fail to identify and destroy our own cells after they become malignant, the result is cancer-cell development and, possibly, the growth of tumors. How can the immune system react to our own cancer cells if the antigens on our cells are supposed to signal "self" to ward off attack? The answer reveals the special mechanism by which our bodies prevent cancer.

Once a cell turns malignant, certain antigens on its surface also change. These altered molecules--known as "cancer-specific" antigens--signal "foreign" to the immune system. Cancer antigens are the giveaway--the slight change in fingerprints that can enable our defenses to detect a dangerous "inside job." Fortunately, our immune cells are not only guards, policemen, and soldiers; they're detectives as well. They have to be, because the outlaw cancer cell often cloaks its identity as a traitor to the community of cells. (These antigens have been found in some cancer types but not all. The search continues, because cancer-specific antigens can be used in vaccines or other immunological approaches to preventing and treting cancer.)

Antibodies: The Keys. Antibodies are the body's complement to antigens. Think of each antigen as a unique lock, and the protein molecules called antibodies (or immunoglobulins) as custom-made keys for every variation of lock. Antibodies are marvels of immunity that can fit into the "keyhole" of any one of millions of different antigens. Each one has a unique molecular configuration, and we can produce antibodes that latch perfectly into every conceivable antigen.

Antibodies are carried throughout the body by white blood cells called lymphocytes, the prime movers of the immune system. Lymphocytes are divided into two main classes--T-lymphocytes and B-lymphocytes (T-cells and B-cells, for short). It is the B-cells that manufacture, display, and secrete antibodies.

Imagine the immune system as a tree with two monumental branches--the cell- mediated arm, led by T-cells, and the humoral arm, led by B-Cells and the antibodies they produce. The T- and B-cell branches themselves have offshoots: sub-populations of cells, cell products, and messengers that enable each branch to carry out its broad goal-- effective destuction of foreign substances. The cell-mediated and humoral branches also communicate with each other for the purposes of a coordinated attack.

B-cells circulate throughout the body with antibody molecules on their surfaces. When they pick up the signal of a particular antigen, they multiply and transform into plasma cells, which are essentially minifactories with one purpose: to churn out the precise antibodies that hook onto the antigens of the interloper. Antibodies not only to neutralize foreign substances or microbes, they signal other immune sentries into battle. An antigen-antibody bond is a call to arms, and a cascade of immune reactions follow the initial "connection." This cascade--the sum total of activity by both branches of the immune system--is called an immune response.

Certain B-cells also remember their encounters with foreign agents. As a result, antibodies are produced swiftly when the same invader attacks again. Immunologic memory is the basis for vaccines, which introduce small amounts of antigen to prime our bodies for subsequent attacks.

T-Cells: The Prime Players: T-cells are the stars of cell-mediated immunity, the branch consisting of subgroups of interacting cells. T-cells are so named because they "grow up" in the thymus, the walnut-sized gland located under the breastbone. Although all immune cells are "born" in the bone marrow, different types follow different developmental pathways. T-cells migrate to the thymus. There, with the aid of various thymic hormones, immature T-cells grow, learn to recognize and attack antigens, and develop a range of specialized activities. The thymus is the master gland of cell-mediated immunity, a veritable training school for different classes of T-cells. Mature T-cells are harbored in the spleen and lymph nodes, waiting there for the sound of an alarm signalling an intruder. As with B-cells, the T-cell line also generates memory cells that prime the bdoy for repeat attacks by a familiar invader. The main subcategories of T-cells include:

  1. T-helper cells orchestrate the actions of other immune cells. They are essential to the performance of their fellow B-cells of the humoral branch; certain antibody reactions depend on help from the helper T's. T-helpers, which are also referred to as CD4 cells (so named for one of their cell-surface receptors), are the primary targets of HIV, the virus that causes AIDS. HIV's destruction of T-helpers, which are crucial conductors of immunity, is the reason why people with AIDS eventually lose their capacity to fight off infections and cancer cells.

  2. Killer T-cells, also known as cytotoxic T-cells, are able to liquidate invading microbes, viruses, or cancer cells. Once alerted by other immune cells, and activated by messenger molecules, the killer T's go into action. They have nimble receptors on their surface that reconfigure their structure to fit snugly into their adversaries' antigens. Once attached, the T-cell injects a load of toxic chemicals into the invader, puncturing its surface membrane and causing its insides to gush out into the fluid environment.

  3. Suppressor T-cells are vital to maintaining properly balanced immune responses. Sometimes called CD8 cells, they are able to suppress or dampen the actions of other immune cells. Without the activity of suppressor Ts, immunity could easily get out of hand, resulting in allergic or autoimmune reactions. But CD8 cells are multi- faceted--they can also destroy virus-infected cells. That's why their strength and numbers are considered crucial to individuals infected by HIV.

Big-Eaters and Non-Specific Fighters: A third class of immune cells are the multi-talented scavenger cells. Also called phagocytes, they engulf microbes or other unwanted products in the bloodstream. The main phagocyte is the macrophage, which literally means "big eater," based on its ability to gobble up foreign substances.

Macrophages, which begin their cellular lives as monocytes, are the garbagemen of the immune system. They clean up waste products in the aftermath of an immune cell attack. But macrophages are also critically involved in the earliest phases of our immune responses. They kick off the immunologic cascade by processing and presenting antigens to lymphocytes, which then initiate full-fledged cellular and humoral reactions. Macrophages also release messenger molecules, such as interleukin- 1, that stimulate and inform lymphocytes while the immune attack ensues. Another product of macrophages, tumor necrosis factor (TNF), is like the body's own chemotherapy--it has the noteworthy ability to liquidate cancer cells.

Immune responses require breathtakingly complex interactions throughout the entire immune network. T-helper cells need antigens presented to them by macrophages, and they depend on numerous signals from other cells and messenger molecules. B-cells depend on T-helpers to do their job, so both branches--cell-mediated and humoral--ultimately depend on macrophages.

Unlike T- and B-cells, macrophages are "non-specific": they don't latch onto invaders in a perfectly targetted "lock-and-key" fashion. But they do swallow up and present invaders to specific T-cells, and clean up the messy aftermath. Another group of non-specific immune cells, from neither B- or T-cell lineages, are the natural killer cells, or "NK cells."

NK cells have the capacity to recognize viruses and cancer cells without having encountered them before, without having antigens served up to them by other cells, and without a specific lock-in-key receptor. Through mechanisms not fully understood, NK cells execute "quick strikes" against virus-infected and cancer cells, killing them with stunning efficiency. In animal studies, NK cells have been shown responsible for stopping the spread of cancer cells throughout the body. Immunologists suspect that NKs serve the same life-saving function in humans, as well.

A vital mind-body connection has been uncovered with NK cells. A multitude of methdologically sound studies have demonstrated relationships between how we cope with stress and the vitality of our NK cells. These cells represent a bridge between psychological factors and our resistance to viral and malignant diseases.

Cell Products and Messenger Molecules: Our immune cells manufacture a vast number of biological products. These are molecules whose functions vary as widely as the scientific names given them: "biological response modifiers," "cytokines," "cell products," "growth factors,""messenger molecules," and just plain "biologicals." Regardless of their titles, these substances carry information and instructions from one group of immune cells to another, changing behavior and coordinating immune responses. These molecules are couriers, communicators, helpers, growth inducers, and suppressors.

Among the most well-known immune-cell products are the interferons, which have antiviral and anticancer properties, and the interleukins, many of which fight cancer, as well. There are many sub-types of interferon and interleukin, each of which perform distinct functions--but all are critical links in the immunologic chain reaction. Scores of other products, each with its own name and properties, regulate the activities of our immune cells.

As mentioned, one of PNI's most surprising discoveries is that brain chemicals- -the neuropeptides and neurotransmitters--also carry messages to immune cells. (Receptors for these brain chemicals have been found on lymphocytes, macrophages, and natural killer cells.) Moreover, recent research has shown that the immune system itself produces neuropeptide-like molecules, and brain cells appear to make immune chemicals, such as interleukin-1. We are just beginning to understand the true reciprocity of the brain-immune dialogue. Brain and body make and receive the same kinds of chemicals in order to communicate effectively. They "speak" the same language--the language of messenger molecules.


Henry Dreher is a health and medical writer specializing in complementary and mind-body medicine. He is the author of Your Defense Against Cancer and The Immune Power Personality, and coauthor of The Type C Connection: The Mind-Body Link to Cancer and Your Health; and Healing Mind, Healthy Woman. Mr. Dreher is a regular contributor to Advances: The Journal of Mind-Body Health, Natural Health, and numerous other periodicals.

From THE IMMUNE POWER PERSONALITY by Henry Dreher. Copyright @ Henry Dreher, 1995. Reprinted by arrangement with Dutton Signet, a division of Penguin Books USA, Inc.
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A note from TheBody.com: Since this article was written, the HIV pandemic has changed, as has our understanding of HIV/AIDS and its treatment. As a result, parts of this article may be outdated. Please keep this in mind, and be sure to visit other parts of our site for more recent information!



  
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This article was provided by Penguin Books USA, Inc.. It is a part of the publication The Immune Power Personality.
 
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