The Immune System / What is The Immune System?

The immune system is a complex network of specialized cells and organs that has evolved to defend the body against attacks by "foreign" invaders. When functioning properly it fights off infections by agents such as bacteria, viruses, fungi, and parasites. When it malfunctions, however, it can unleash a torrent of diseases, from allergy to arthritis to cancer to AIDS.

The immune system evolved because we live in a sea of microbes. Like man, these organisms are programmed to perpetuate themselves. The human body provides an ideal habitat for many of them and they try to break in; because the presence of these organisms is often harmful, the body's immune system will attempt to bar their entry or, failing that, to seek out and destroy them.

The immune system, which equals in complexity the intricacies of the brain and nervous system, displays several remarkable characteristics. It can distinguish between "self" and "nonself." It is able to remember previous experiences and react accordingly; thus, once you have had chicken pox, your immune system will prevent you from getting it again. The immune system displays both enormous diversity and extraordinary specificity; not only is it able to recognize many millions of distinctive nonself molecules, it can produce molecules and cells to match up with and counteract each one of them. And it has at its command a sophisticated array of weapons.

The success of this system in defending the body relies on an incredibly elaborate and dynamic regulatory-communications network. Millions and millions of cells, organized into sets and subsets, pass information back and forth like clouds of bees swarming around a hive. The result is a sensitive system of checks and balances that produces an immune response that is prompt, appropriate, effective, and self-limiting.

The organs involved with the immune system are called the lymphoid organs, which affect growth, development, and the release of lymphocytes (a certain type of white blood cell). The blood vessels and lymphatic vessels are important parts of the lymphoid organs, because they carry the lymphocytes to and from different areas in the body. Each lymphoid organ plays a role in the production and activation of lymphocytes. Lymphoid organs include:

• adenoids (two glands located at the back of the nasal passage)
• blood vessels (the arteries, veins, and capillaries through which blood flows)
• bone marrow (the soft, spongy tissue found in bone cavities)
• lymph nodes (small organs shaped like beans, which are located throughout the body and connect via the lymphatic vessels)
• lymphatic vessels (a network of channels throughout the body that carries lymphocytes to the lymphoid organs and bloodstream)
• Peyer’s patches (lymphoid tissue in the small intestine)
• (a fist-sized organ located in the abdominal cavity)
• thymus (two lobes that join in front of the trachea behind the breast bone)
• tonsils (two oval masses in the back of the throat)

At the heart of the immune system is the ability to distinguish between self and nonself. Virtually every body cell carries distinctive molecules that identify it as self.

The body's immune defenses do not normally attack tissues that carry a self marker. Rather, immune cells and other body cells coexist peaceably in a state known as self-tolerance. But when immune defenders encounter cells or organisms carrying molecules that say "foreign," the immune troops move quickly to eliminate the intruders.

Any substance capable of triggering an immune response is called an antigen. An antigen can be a virus, a bacterium, a fungus, or a parasite, or even a portion or product of one of these organisms. Tissues or cells from another individual, except an identical twin whose cells carry identical self-markers, also act as antigens; because the immune system recognizes transplanted tissues as foreign, it rejects them. The body will even reject nourishing proteins unless they are first broken down by the digestive system into their primary, non-antigenic building blocks.

An antigen announces its foreignness by means of intricate and characteristic shapes called epitopes, which protrude from its surface. Most antigens, even the simplest microbes, carry several different kinds of epitopes on their surface; some may carry several hundred. However, some epitopes will be more effective than others at stimulating an immune response.

In abnormal situations, the immune system can wrongly identify self as nonself and execute a misdirected immune attack. The result can be a so-called autoimmune disease such as rheumatoid arthritis or systemic lupus erythematosus.

In some people, an apparently harmless substance such as ragweed pollen or cat hair can provoke the immune system to set off the inappropriate and harmful response known as allergy; in these cases the antigens are known as allergens.

Molecules that mark a cell as self are encoded by a group of genes that is contained in a sections of a specific chromosome known as the major histocompatibility complex (MHC). The prefix "histo" means tissue; the MHC was discovered in the course of tissue transplantation experiments. Because MHC genes and the molecules they encode vary widely in the details of their structure from one individual to another (a diversity known as polymorphism), transplants are very likely to be identified as foreign and rejected by the immune system.

Scientists eventually discovered a more natural role for the MHC: it is essential to the immune defenses. MHC markers determine which antigens an individual can respond to, and how strongly. Moreover, MHC markers allow immune cells such as B cells, T cells, and macrophages to recognize and communicate with one another.

One group of proteins encoded by the genes of the MHC are the markers of self that appear on almost all body cells. Known as class I MHC antigens, these molecules alert killer T cells to the presence of body cells that have been changed for the worse-infected with a virus or transformed by cancer-and that need to be eliminated.

A second group of MHC proteins, class II antigens, are found on B cells, macrophages, and other cells responsible for presenting foreign antigen to helper T cells. Class II products combine with particles of foreign antigen in a way that showcases the antigen and captures the attention of the helper T cell.

This focusing of T cell antigen recognition through class I and class II molecules is known as MHC (or histocompatibility) restriction.

The organs of the immune system are stationed throughout the body. They are generally referred to as lymphoid organs because they are concerned with the growth, development, and deployment of lymphocytes, the white cells that are the key operatives of the immune system. Lymphoid organs include the bone marrow and the thymus, as well as lymph nodes, spleen, tonsils and adenoids, the appendix, and clumps of lymphoid tissue in the small intestine known as Peyer's patches. The blood and lymphatic vessels that carry lymphocytes to and from the other structures can also be considered lymphoid organs.

Cells destined to become immune cells, like all other blood cells, are produced in the bone marrow, the soft tissue in the hollow shafts of long bones. The descendants of some so-called stem cells become lymphocytes, while others develop into a second major group of immune cells typified by the large, cell-and particle-devouring white cells known as phagocytes.

The two major classes of lymphocytes are B cells and T cells. B cells complete their maturation in the bone marrow. T cells, on the other hand, migrate to the thymus, a multilobed organ that lies high behind the breastbone. There they multiply and mature into cells capable of producing immune response-that is, they become immunocompetent. In a process referred to as T cell "education," T cells in the thymus learn to distinguish self cells from nonself cells; T cells that would react against self antigens are eliminated.

Upon exiting the bone marrow and thymus, some lymphocytes congregate in immune organs or lymph nodes. Others-both B and T cells-travel widely and continuously throughout the body. They use the blood circulation as well as a bodywide network of lymphatic vessels similar to blood vessels.

Laced along the lymphatic routes-with clusters in the neck, armpits, abdomen, and groin-are small, bean-shaped lymph nodes. Each lymph node contains specialized compartments that house platoons of B lymphocytes, T lymphocytes, and other cells capable of enmeshing antigen and presenting it to T cells. Thus, the lymph node brings together the several components needed to spark an immune response.

The spleen, too, provides a meeting ground for immune defenses. A fist-sized organ at the upper left of the abdomen, the spleen contains two main types of tissue: the red pulp that disposes of worn-out blood cells and the white pulp that contains lymphoid tissue. Like the lymph nodes, the spleen's lymphoid tissue is subdivided into compartments that specialize in different kinds of immune cells. Microorganisms carried by the blood into the red pulp become trapped by the immune cells known as macrophages. (Although people can live without a spleen, persons whose spleens have been damaged by trauma or by disease such as sickle cell anemia, are highly susceptible to infection; surgical removal of the spleen is especially dangerous for young children and the immunosuppressed.)

Nonencapsulated clusters of lymphoid tissue are found in many parts of the body. They are common around the mucous membranes lining the respiratory and digestive tracts-areas that serve as gateways to the body. They include the tonsils and adenoids, the appendix, and Peyer's patches.

The lymphatic vessels carry lymph, a clear fluid that bathes the body's tissues. Lymph, along with the many cells and particles it carries-notably lymphocytes, macrophages, and foreign antigens, drains out of tissues and seeps across the thin walls of tiny lymphatic vessels. The vessels transport the mix to lymph nodes, where antigens can be filtered out and presented to immune cells.

Additional lymphocytes reach the lymph nodes (and other immune tissues) through the bloodstream. Each node is supplied by an artery and a vein; lymphocytes enter the node by traversing the walls of the very small specialized veins.

All lymphocytes exit lymph nodes in lymph via outgoing lymphatic vessels. Much as small creeks and streams empty into larger rivers, the lymphatics feed into larger and larger channels. At the base of the neck, large lymphatic vessels merge into the thoracic duct, which empties its contents into the bloodstream.

Once in the bloodstream, the lymphocytes and other assorted immune cells are transported to tissues throughout the body. They patrol everywhere for foreign antigens, then gradually drift back into the lymphatic vessels, to begin the cycle all over again.

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