The human body must always protect itself against foreign invasion. A complex defense system is in place to counterattack such invasions. The immune system in humans consists of nonspecific defense responses ( eg, phagocytosis) and specific immune responses (eg, humoral and cell-mediated immunity). Immunity is the body’s ability to recognize and destroy specific pathogens and to prevent infectious diseases. When the immune system is compromised or unable to respond, immunodeficiency diseases and malignancies occur. When the immune system is overreactive, hypersensitivity disorders, such as allergies and autoimmune diseases, result ( Lewis, Collier, & Heitkemper, 1996)
STRUCTURE AND FUNCTION
The body’s immune system includes the bone marrow, lymphoid organs, and the mononuclear phagocyte system (also called the reticuloendothelial system). Primary functions of the immune system include defense, homeostasis, and surveillance. Bax 24-1 describes these functions. Fig 24-1 indicates the spesific organs involved in the imune system.
Bone marrow and lymphocyte production
The cell in the bone marrow are capable of developing into any of three types of blood cells: erythrocytes ( red blood cells or RBCs), leukocytes ( white bloode cells or WBCs) or thrombocytes (platelets). WBCs defend the body againts disease organisms, toxin, and irritants. The two types of WBCs are granular (neutrophils, basophil, and eosinophils) and agranular ( monocytes and lymphocytes). Lymphocytes are the “ cornerstone” of the immne system-they alone have ability ti recognize foreign substances in the body. Differentiation of the lymphocytes into special lymphocytes called B cells ( B lymphocytes) and T cells (T lymphocytes) must occue before detection of foreign invaders begins.
ANTIGENS AND ANTIBODIES
An antigens is any foreign substance entering the body that stimulates an immune response ( the activity of B or T lymphocytes). The most antigens are large protein molecules found on the surface of foreign organisms, RBCs, and tissue cells; on pollen ; and in toxins and foods. Some carbohydrates and lipids also act as antigens (Memmler, Cohen, & Wood, 1996).
An antibody (Ab) is a protein substance the body produces in response to an antigen. B lymphocytes are responsible for antibody production. All antibodies are contained in a portion of the blood plasma called the gamma globulin fraction. Thus, antibodies are sometimes referred to as immunoglobulins (Ig) (Memmler et al. 1996 b). The five groups antibodies are IgG, IgA, IgM, IgD, IgE (see p. 261). Each antigen (foreign invader) stimulates the production of its own specifis antibody. The body can make about 1 million individual antibodies. Antibodies do not destroy antigens. They label antigens for destruction.
B LYMPHOCYTES
Bone marrow is responsible not only for the production of B lymphocytes, but also for their maturation. Once mature, B cells can become activated in the blood. Exposure to an antigen in the bloodstream activites B cells to enlarge and to multiply rapidly to produce colonies of clones, althought B cells do notbrespond to all pathogens. Most of these clones become plasma cells that produce specific antibodies to circulate in the blood. These antibodies provide the form of immunity called humoral immunity ( humoral =body fluid). In the process of humoral immunity, macrophages destroy antigens after antibodies have identified them for destruction. Those clones that do not become plasma cells remain in the body as memory cells. On repeated exposure to an antigen, the memory cells are ready to produce antibodies immediately. This “ Immunologic memory” makes aperson immune to reinfection with a disease once he or she has had it (Memmler et all,. 1996b) b lymphocytes are found predominantly in organized lymphoid tissues, such as the spleen. They constitute only about 10 % to 20% of the circulating lymphocytes in the tissues and the bloode. Even fewer B cells are found in the lymph ( Burrell, Gerlach, & Plesss, 1997)
T LYMPHOCYTES
Some immature stem cells that the bone marrow produces migrates to the thymus gland to become T cells ( thymu-derived lynphocytes). T cells make up the remaining 80% to 90% of lymphocytes found in the circulating blood. While in the thymus, T cells profilerate and become sensitized, or capable of combining with spesific foreign antigens. T lymphocytes produce an immunity called cell-mediated immunity.
T lymphocytes are generally responsible for fighting cancer cells, viruses, and intracelular parasites. They are also responsible for tissue rejection during organ transplantation. Several types of T lymphocytes exist, each of which has its own fuction. Table 24-1 identifies the types and associated function of the T lymphocytes.
For a T cell to react with a specific antigen, the antigen must first be presented to the T cell on the surface of a macrophage. Macrophage, when combined with T cells release substance called interleukins, which stimulate T cell growth ( Memmleret al,. 1996)
STRUCTURE AND FUNCTION
The body’s immune system includes the bone marrow, lymphoid organs, and the mononuclear phagocyte system (also called the reticuloendothelial system). Primary functions of the immune system include defense, homeostasis, and surveillance. Bax 24-1 describes these functions. Fig 24-1 indicates the spesific organs involved in the imune system.
Bone marrow and lymphocyte production
The cell in the bone marrow are capable of developing into any of three types of blood cells: erythrocytes ( red blood cells or RBCs), leukocytes ( white bloode cells or WBCs) or thrombocytes (platelets). WBCs defend the body againts disease organisms, toxin, and irritants. The two types of WBCs are granular (neutrophils, basophil, and eosinophils) and agranular ( monocytes and lymphocytes). Lymphocytes are the “ cornerstone” of the immne system-they alone have ability ti recognize foreign substances in the body. Differentiation of the lymphocytes into special lymphocytes called B cells ( B lymphocytes) and T cells (T lymphocytes) must occue before detection of foreign invaders begins.
ANTIGENS AND ANTIBODIES
An antigens is any foreign substance entering the body that stimulates an immune response ( the activity of B or T lymphocytes). The most antigens are large protein molecules found on the surface of foreign organisms, RBCs, and tissue cells; on pollen ; and in toxins and foods. Some carbohydrates and lipids also act as antigens (Memmler, Cohen, & Wood, 1996).
An antibody (Ab) is a protein substance the body produces in response to an antigen. B lymphocytes are responsible for antibody production. All antibodies are contained in a portion of the blood plasma called the gamma globulin fraction. Thus, antibodies are sometimes referred to as immunoglobulins (Ig) (Memmler et al. 1996 b). The five groups antibodies are IgG, IgA, IgM, IgD, IgE (see p. 261). Each antigen (foreign invader) stimulates the production of its own specifis antibody. The body can make about 1 million individual antibodies. Antibodies do not destroy antigens. They label antigens for destruction.
B LYMPHOCYTES
Bone marrow is responsible not only for the production of B lymphocytes, but also for their maturation. Once mature, B cells can become activated in the blood. Exposure to an antigen in the bloodstream activites B cells to enlarge and to multiply rapidly to produce colonies of clones, althought B cells do notbrespond to all pathogens. Most of these clones become plasma cells that produce specific antibodies to circulate in the blood. These antibodies provide the form of immunity called humoral immunity ( humoral =body fluid). In the process of humoral immunity, macrophages destroy antigens after antibodies have identified them for destruction. Those clones that do not become plasma cells remain in the body as memory cells. On repeated exposure to an antigen, the memory cells are ready to produce antibodies immediately. This “ Immunologic memory” makes aperson immune to reinfection with a disease once he or she has had it (Memmler et all,. 1996b) b lymphocytes are found predominantly in organized lymphoid tissues, such as the spleen. They constitute only about 10 % to 20% of the circulating lymphocytes in the tissues and the bloode. Even fewer B cells are found in the lymph ( Burrell, Gerlach, & Plesss, 1997)
T LYMPHOCYTES
Some immature stem cells that the bone marrow produces migrates to the thymus gland to become T cells ( thymu-derived lynphocytes). T cells make up the remaining 80% to 90% of lymphocytes found in the circulating blood. While in the thymus, T cells profilerate and become sensitized, or capable of combining with spesific foreign antigens. T lymphocytes produce an immunity called cell-mediated immunity.
T lymphocytes are generally responsible for fighting cancer cells, viruses, and intracelular parasites. They are also responsible for tissue rejection during organ transplantation. Several types of T lymphocytes exist, each of which has its own fuction. Table 24-1 identifies the types and associated function of the T lymphocytes.
For a T cell to react with a specific antigen, the antigen must first be presented to the T cell on the surface of a macrophage. Macrophage, when combined with T cells release substance called interleukins, which stimulate T cell growth ( Memmleret al,. 1996)
Lymphoid organs
The lymphatic system plays a large role in defending and protecting the body against disease. Important lymphatic organs that function in immunity include the bone marrow, thymus, lymph node, and spleen.
PRIMARY LYMPHOID ORGANS
Along with the bone marrow, the thymus is considered a “central“ lymphoid organs. This small gland weighs 1 ounce at most. It is located in the mediastinum of the upper chest. The thymus is most active early in life and begins to atrophy at puberty. T lymphocytes must mature in thymus gland before they can perform their imune function. The thymus produces hormones called thymosin, thymic humoral factor (THF), thymic factor (TF) and Thymopoietin. These hormones promote the proliferation and maturation of T cells in the thymus and other lymphoid tissues throughout the body.
PERIPHERAL (SECONDARY) LYMPHOID ORGANS
The peripheral or secondary lymphoid organs of the immune system inculde the lymphoid structures scattered in the submucosal layers of the respiratory, gastrointestinal, and genitourinary tracts; the lymph nodes; and the spleen. The defense function of these organs are primarily related to the filtration of tisssue fluid or lymph and protection against external microorganisms. Chapter 23 present spesific descriptions of these peripheral lymphoid organs.
The Mononuclear Phagocyte System
The mononuclear phagocyte system, or reticuloentheal system, consist of spesialized cells throughout the body that can ingest particulate matter. These cells begin as monocytes and transform into macrophages (phagocytes cells) after entering other tissues via the bloodstream. The system is concered with the destruction of worn-out blood cells, bacteria, cancer cell, and other foreign substances dangerous to the body. Some macrophage have special names, such as kupffer cells, in the liver sinusoid or dust cells in the lung.
Mononuclear phagocytes play a very important role in both nonspecific immunity. In specific immunty they are responsible for capturing ( via phagocytosis), prosessing, and presenting the antigen to the lymphocytes. The macrophage-bound antigen, when presented to the B or T lymphocyte, triggers the humoral or cell-mediated immune response ( Lewis et al,. 1996)
SYSTEM PHYSIOLOGY
The body possesses several defense mechanisms. Nonspecific defense systems ( sometimes called nonspecific immunity) fight against a variety of foreign invaders. This unit presents several nonspecific defense mechanisms:
• The skin provides a physical barrier and secretes enzymes that kill bacteria
• Tears dilute and wash away irritating substances and microbes
• Neutrophils and monocytes ingest and destroy bacteria and toxins and remove cellular debris
• Interferon is a protein made by several types of cells that inhibits virus production and infection
• Fever intensifies the effects of interferons, inhibits the growth of some microbes, and speeds up body reactions that aid repair.
• The respiratory tract contains cilia and macrophages (phagocytic cells) in its mucous membrane lining that trap and remove microbes and dust.
• The stomach contains hydrochloric acid, which destroy pathogens.
• Vomiting defecation and urination expel microbes from the body.
Specific immunity is considered the final line of defense against desease. Specific defense mechanisms are able to recognize and to respond to specific substances. Humoral and cells- mediated immunity are considered specific defense mechanisms because they act against particular harmful substance.
Types Of Specific Immunity
Specific immunity can be classified into two main categories: inborn and acquired. Inborn immunity refers to immunity that is inherited. Acquired immunity is obtained through natural or artifical sources. Both natural and artificial immunitycan also be attained actively or passively ( table 24-2) . Individuals develop acquired immunity during their lives as they are exposed to desease- causing organisms. In other word, acquired immunity is built on lifetime “ exposures”
Natural acquired immunity occurs when a person is not deliberately exposed to a causative agent. This immunity can occur both actively and passively. The active type results when a child is exposed to and develops a disease (eg, measles or chikenpox) and subsequently builds up immunity to infections caused by the same organism. Remember that the body manufactures both cells that target the infecting antigen and memory cells. Each time the person is exposed to the disease, the memory cells activate a response that produces antibodies to the offending antigen. Naturally acquired immunity can last a view years or for a lifetime.
Passively acquired natural immunity occurs during pregnancy across the shared placenta. After birth, the baby can receive protection through the mother’s breast milk. This protection last up to 6 months, when the infant’s own immune system begins to take over. igG may also be injected after disease exposure for short-term protection.
Artificial accquired immunity occurs when a person is deliberately exposed to a causatives agent ( immunization). It also can be acquired through active or passive means. The active type occures through an injection of the causative agent into a person’s system called a vaccine. The causative agent is diluted to reduce its virulence to allow antibodies to form without causing illness. Examples of vaccine are those for pertussis (whooping cough), measles, and mumps.
The passive type occurs with the injection of antibodies into a person’s system that were produced by another individual’s immune system. Examples of this type of immunity include immunization for hepatitis, rabies and tetanus.
Antigen-Antibody Reaction
Antigen-Antibody reactins begin with the B lymphocytes whose job is to produce hunoral immunity is the body’s resistance to circulating disease-producing antigens and bacteria. B cells become plasma cells, and then work to produce antibodies.
Antibody- mediated immunity changes an antigen, rendering it harmlessbto the body. The antibody complishs this task by bingding to an antigen, forming an antigen-antibodu complex. This binding can be compared to a “ lock-and key´mechanism. In the same way that keys are cutindividually to fit a particular lock and no other, an antibody forms in response to a specific antigen the patterns on the membrane surface on the antigen and antibody fit togetherwith the same ciscrimination that a keyfits into alock. The match must be perfect.
Once attached to the antigen, the antibody eses one of several mechanisms to disarm the antigen. The body can neutralize the antigen’s toxins. Another mechanisms is to cause harmful cells to clump togther so macrophages and destroy them. Antibodies promote or enhance phagocytosis by helpng phagocytes attach to the cells they will destroy.
Another mechanism for antigen destruction is called complement fixation. A complement is a group is proteiins normally present but inactive in the blood. Complements become active when exposed to the altered cellular shape caused by the antigen – antibody complex. When actived, compplements cause the formation of highlyspesialized antigen-antibody complexes that target specific cells. These newly formed complexes casue holes to develop in cells membranes. Sodium and water flow into the cells, causing them to brust open, and thus be destroyed.
Antibodies are commonly called gamma globulins immunoglobulins. The five basic groups are:
• IgG – protects the fetus before birth against antitoxins, viruses and bacteria.
• IgM- stimulate complement activity
• igA – protecs mucosal surfaces
• IgE – responsible are allergic reaction
• IgD – acts as an antigen receptor
EFFECT OF AGING SYSTEM
Older adult have fewer T cells and B cells. The T cells and B cells that remain function poorly as stem cells. Consequently, the immune system of the older adult acts with a slower, muted responsevto infection and to inflammatory procces. Older adult usually have baseline body temperature lower than 98.6 F. Thus, they do not always have febrile response to infection.
Infection frequently manifest themselves in older adult as a change in metal status. The cardiovaskuler system can not keep up with the increased metabolic demands an unrecognized infection imposes. The result is cerebral hypoxia or “delirium”. Because of this athypical presentation, older adult are susceptible to developing bacteremia ( bacteri in the blood) that, if untreated, can quickly progress to septic shock.
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