The immune system is a network of cells and organs that work together to defend the body against attacks by "foreign" invaders. These are primarily germs-tiny, infection-causing organisms such as bacteria and viruses, as well as parasites and fungi. Because the human body provides an ideal environment for many germs, or microbes, they try to break in. It is the immune system's job to keep them out or, failing that, to seek them out and destroy them.
When the immune system misfires, however, or when it is crippled, it can unleash a torrent of diseases-allergy or arthritis or cancer or AIDS.
The immune system is amazingly complex. It can recognize millions of different enemies, and it can produce secretions and cells to match up with and wipe out each one of them.
The secret to its success is an elaborate and dynamic communications network: millions and millions of cells, organized into sets and subsets, pass infromation back and forth like clouds of bees swarming around a hive. Once immune cells receive the alarm, they undergo strategic changes and begin to produce powerful chemicals. These substances allow the cells to regulate their own growth and behavior, enlist their fellows, and direct new recruits to trouble spots.
At the heart of the immune system is a remarkable ability to distinguish between the body's own cells-self-and foreign cells- nonself. The body's immune defenses normally coexist peacefully with cells that carry distinctive "self" marker molecules. But when immune defenders encounter cells or organisms carrying markers that say "foreign," they quickly swing into action.
Anything that can trigger this immue response is called an ANTIGEN. An antigen can be a germ such as a virus, or even a part of a virus. Tissues or cells from another person (except an identical twin) also carry nonself markers and act as antigens; this explains why tissue transplants are rejected. The body will even reject nourishing proteins in food unless they are first broken down by the digestive system.
In abnormal situations, the immune system can mistake self for nonself and attackt it; the result is called autoimmune disease. Some forms of arthritis and diabetes are autoimmune diseases. In other cases the immune system responds inappropriately to a seemingly harmless substance such as ragweed pollen or cat hair; the result is allergy, and this kind of antigen is called an ALLERGEN.
The organs of the immune system are positioned throughout the body. They are called LYMPHOID ORGANS because they are home to LYMPHOCYTES, small white blood cells that are the key players in the immune system.
BONE MARROW, the soft tissue in the hollow center of bones, is the ultimate source of all blood cells, including white blood cells destined to become immune cells. The THYMUS is an organ that lies behind the breastbone; lymphocytes known as T CELLS MATURE in the thymus.
Lymphocytes can travel throughout the body, using either the BLOOD VESSELS or their own system of LYMPHATIC VESSELS. Like small creeks that empty into larger and larger rivers, the lymphatic vessels feed into larger and larger channels. At the base of the neck they merge into a large duct, which discharges its contents into the bloodstream. The lymphatic vessels carry LYMPH, a clear fluid that bathes the body's tissues.
Small, bean-shaped LYMPH NODES are laced along the lymphatic vessels with clusters in the neck, armpits, abdomen, and groin. Each lymph node contains specialized compartments where immune cells congregate, and where they can encounter antigens.
The SPLEEN is a fist-sized organ at the upper left of the abdomen. Like the lymph nodes, the spleen contains specialized compartments where immune cells gather and work, and serves as a meeting ground where antigens confront the immune defenses.
Clumps of lymphoid tissue are found in many parts of the body, especially in the linings of the digestive tract and the airways and lungs-territories that serve as gateways to the body. These tissues include the TONSILS, the ADENOIDS, and the APPENDIX.
Immune cells and foreign particles enter the lymph nodes via incoming lymphatic vessels or the lymph nodes' tiny blood vessels. All lymphocytes exit lymph nodes through outgoing lymphatic vessels. Once in the bloodstream, they are transported to tissues throughout the body. They patrol everywhere for foreign antigens, then gradually drift back into the lymphatic system, to begin the cycle all over again.
The immune system stockpiles a huge arsenal of cells, not only lymphocytes but also cell-devouring phagocytes and their relatives. Some immune cells take on all comers, while others are trained on highly specific targets. To work effectively, most immune cells need cooperation of their fellows. Sometimes immune cells communicate by direct physical contact, sometimes by releasing chemical messengers.
In order to have room for all the cells needed to match millions of possible enemies, the immune system stores just a few of each kind. When an antigen appears, those few matching cells multiply into a full- scale army. After their job is done, they fade away.
Lymphocytes are one of the main types of immune cells. And B cells and T cells are the main types of lymphocytes.
B CELLS work chiefly by secreting soluble substances called ANTIBODIES into the body's fluids. Antibodies ambush antigens circulating in the bloodstream. However, they are powerless to penetrate cells. The job of attacking target cells- either cells that have been infected by viruses or cells that have been distorted by cancer-is left to T lymphocytes or other immune cells (described below).
Each B cell is prorammed to make one specific antibody. For example, one B cells will make an antibody that blocks a virus that causes the common cold, while another produces an antibody that attacks a bacterium that causes pneumonia.
When a B cell encounters its triggering antigen, it gives rise to many large cells known as plasma cells. Every plasma cell is essentially a factory for producing antibody. Each of the plasma cells descended from a given B cell manufactures millions of identical antibody molecules and pours them into the bloodstream.
An antibody matches an antigen much as a key matches a lock. Some match exactly; others fit more like a skeleton key. But whenever antibody and antigen interlock, the antibody marks the antigen for destruction.
Antibodies belong to a family of large molecules known as IMMUNOGLOBULINS. Different types play different roles in the immune defense strategy. Immunoglobulin G, or IgG, works efficiently to coat microbes, speeding their uptake by other cells in the immune system. Immunoglobulin M is very effective in killing bacteria. Immunoglobulin A concentrates in body fluids--tears, saliva, the secretions of the respiratory tract and the digestive tract--guarding the entrances to the body. Immunoglobulin E, whose natural job probably is to protect against parasite infections, is the villain responsible for the symptoms of allergy.
T cells contribute to the immune defenses in two major ways. Some direct and regulate the immune responses. Others are killer cells that attack cells that are infected or cancerous. Less helpfully, killer T cells assail foreign cells transplanted as organ grafts.
T lymphocytes work primarily by secreting potent chemical messengers known as CYTOKINES or, more specifically, LYMPHOKINES. Binding to target cells, lymphokines mobilize many other cells and substances. They encourage the growth of cells, trigger cell activity, direct cell traffic, destroy target cells, and arouse phagocytes.
NATURAL KILLER CELLS (NK cells) are another kind of lethal white cell, or lymphocyte. Like killer T cells, NK cells are armed with granules filled with potent chemicals. But killer T cells attack only their specific matching targets; natural killer cells attack any foe.
Both kind of killer cells slay on contact. The deadly assassin binds to its target, aims its weapons, and then delivers a lethal burst of chemicals.
Note the Observation that Killer T-Cell Activity may be Dis-armed by HIV
PHAGOCYTES (or "cell eaters") are large white cells that can swallow and digest microbes and other foeign particles. MONOCYTES are phagocytes that circulate in the blood. When monocytes migrate into tissues, they develop into MACROPHAGES, or "big eaters." Specialized types of macrophages can be found in many organs, including the lungs, the kindneys, the brain, and the liver.
Macrophages play many roles. As scavengers, they rid the body of worn-out cells and other debris. They display bits of foreign antigen in a way that draws the attention of matching lymphocytes. And they churn out an amazing variety of powerful cytokines, known as MONOKINES, which are vital to the immune responses.
GRANULOCYTES are another kind of immune cell. Granulocytes are white blood cells that contain granules filled with potent chemicals, which allow the granulocytes to destroy microorganisms. Some of these chemicals such as histamine also contribute to inflammation and allergy.
One type of granulocyte, the NEUTROPHIL, is also a phagocyte; it uses its prepackaged chemicals to degrade the microbes it ingests. EOSINOPHILS and BASOPHILS are granulocytes that "degranulate," spraying their chemicals into harmful cells or microbes nearby.
The MAST CELLS is a twin of the basophil, except that it is not a blood cell. Rather, it is found in the lungs, skin, tongue, and linings of the nose and intestinal tract, where it is responsible for the symptoms of allergy.
A related structure is a cell fragment, the blood PLATELET. Platelets, too, contain granules. In addition to promoting blood clotting and wound repair, platelets activate some of the immune defenses.
The COMPLEMENT SYSTEM is made up of about 25 body chemicals that work together to "complement" the action of antibodies in destroying bacteria. Complement also helps to rid the body of antibody-coated antigens (ANTIGEN-ANTIBODY COMPLEXES).Complement proteins, which cause blood vessels to become dialated and then leaky, contribute to the redness, warmth, swelling, pain, and loss of function that characterize an INFLAMMATORY RESPONSE.
Complement proteins circulate in the blood in an inactive form. When the first protein in the complement series is activated-- typically by antibody that has locked into an antigen protruding from a cell--it sets in motion a domino effect. Each component takes its turn in a precise chain of steps known as the "complement cascade." The end product is a cylinder inserted into--and puncturing a hole in-- the cells's wall. With fluids and molecules flowing in and out, the cell swells and bursts.
Microbes attempting to get into the body must first move past the body's external armor. The skin and the membranes lining the body's gateways not only pose a physical barrier, they are also rich in scavenger cells and IgA antibodies.
Next, invaders must escape a series of NONSPECIFIC defenses, which are ready to attack, without regard for any specific antigen markers. These include patrolling scavernger cells, natural killer (NK) cells, and complement.
Microbes that cross the nonspecific barriers must then confront specific weapons tailored just for them. Specific weapons, which include both antibodies and cells, are equipped and with singular receptor structures that allow them to recognize and interact with their designated targets.
Note the Observation that Killer T-Cell Activity may be Dis-armed by HIV
Note the Observation that Killer T-Cell Activity may be Dis-armed by HIV
Antibodies are triggered when a B cell encounters its matching antigen. The B cell takes in the antigen and digests it, then displays antigen fragments bound to its own distinctive marker molecules. The combination of antigen fragment and marker molecule attracts the help of a mature, matching T cell. Lymphokines secreted by the T cell allow the B cell to multiply and mature into antibody-producing plasma cells. Released into the bloodstream, antibodies lock onto matching antigens. These antigen-antibody complexes are soon eliminated, either by the complement cascade or by the liver and the spleen.
T cells are mobilized when they encounter a cell such as a macrophage or a B cell that has digested an antigen and is displaying antigen fragments bound to its marker molecules. Lymphokines help the T cells to mature. The T cell, alerted and activated, secretes lymphokines. Some lymphokines spur the growth of more T cells. Some T cells become killer cells and track down body cells infected by viruses. Some lymphokines atract immune cells--fresh macrophages, granulocytes, and other lymphocytes--to the site of infection. Yet other lymphokines direct the recruits once they arrive on the scene.
Long ago physicians realized that people who had recovered from the plague would never get it again--they had acquired immunity. This is because, whenever T cells and B cells are activated, some of the cells become MEMORY CELLS. The next time that an individual meets up with the same antigen, the immune system is set to demolish it.
Immunity can be strong or weak, short-lived or long-lasing, depending on the type of antigen, the amount of antigen, and the route by which it enters the body. Immunity can also be influenced by the genes you inherit; when faced with the same antigen, some individuals will respond forcefully, other feebly, and some not at all.
An immune response can be sparked not only by infection but also by immunization with vaccines. VACCINES contain microorganisms--or parts of microorganisms--that have been treated so they will be able to provoke an immune response but not full-blown disease.
Immunity can also be transferred from one individual to another by injections of serum rich in antibodies (ANTISERUM). Immune serum globulin or "gamma globulin" is sometime given to protect travelers to countries where hepatitis is widespread, but such "passive immunity" typically lasts only a few weeks or months.
Infants, who are born with weak immune responses, are protected for the first few months of life by antibodies they received from their mothers before birth. Babies who are nursed can also receive some antibodies from breast milk; these help to protect the digestive tract.
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