Other Lymphocytes
A natural killer (NK) cell is slightly larger than a B or a T cell and is considered “nonspecific” in immune response. These specialized lymphocytes kill certain microbes (particularly viruses) and cancer cells. They are called natural because they do not require the maturation and “education” of B cells and T cells but are ready to target specific cells as soon as they are produced. Thus, they are part of the body’s natural defense against cancer. NK cells also secrete some cytokines.
Key Concept Killer T cells only recognize antigens that are combined with a molecule called MHC I (major histocompatability complex, class I), which coats the molecule with an antibody HelperT cells only recognize antigens combined with MCH, class II molecules. A minor subtype of T cells recognizes intact antigens not bound to the MHC receptors.
Cytokines are proteins that act as messengers to help regulate some of the functions of lymphocytes and macrophages during the immune response. Some cytokines are given by injection to treat specific diseases.
Cytokines that have been identified are:
• Interferon-alpha: used to treat certain cancers, such as hairy cell leukemia
• Interferon-beta: believed to be helpful in multiple sclerosis
• Interleukin-1: produced by macrophages, mobilizes T lymphocytes
• Interleukin-2: produced by T cells, stimulates production of interferon. Used to treat many solid cancers, such as malignant melanoma and kidney cancer. (Also has adverse effects)
• Interleukin-3: required for differentiation of certain T cells
• Interleukin-8: guides neutrophils to the source of an antigen
• Interleukin-12: stimulates natural killer cells
• Granulocyte colony-stimulating factor: used to help increase neutrophils in clients who are undergoing chemotherapy.
Key Concept When NK cells are incubated with a substance called interleukin-2, they are called lymphokine-activated killer T cells. These cells then function even more effectively than do NK cells as a biological treatment for cancer Pathogens have the ability to mutate rapidly thus avoiding the immune system’s defenses.
LYMPHOID ORGANS
Primary (Central) Lymphoid Organs
In addition to the bone marrow, the thymus is considered a central or primary lymphoid organ. 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 (shrink and die) at puberty. T lymphocytes mature in the thymus gland in children (and in other lymphoid tissues as well in adults) before they can perform their immune functions. 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.
Peripheral (Secondary) Lymphoid Organs
The peripheral or secondary lymphoid organs of the immune system include the lymphoid structures scattered in the submucosal layers of the respiratory, gastrointestinal, and genitourinary tracts as well as the tonsils, lymph nodes, and spleen. The defense functions of these organs are primarily related to the filtration of tissue fluid or lymph for foreign particles and external microorganisms.
THE MONONUCLEAR PHAGOCYTE SYSTEM
The mononuclear phagocyte system, or reticuloendothelial system, consists of specialized cells throughout the body that can ingest foreign particulate matter. These cells begin as monocytes and transform into macrophages (phagocytic or endocytic cells) after entering other tissues via the bloodstream. This system is concerned with the destruction of worn-out blood cells, bacteria, cancer cells, and other dangerous foreign substances. Some macrophages have special names, such as Kupffer cells in the liver sinusoids and dust cells in the lungs.
Mononuclear phagocytes play a very important role in both specific and non-specific immunity. In specific immunity they are responsible for capturing (via phagocytosis), processing, and presenting the antigen to lymphocytes for destruction. The macrophage-bound antigen, when presented to the B or T lymphocyte, triggers the humoral or cell-mediated immune response.
System Physiology
NON-SPECIFIC DEFENSE MECHANISMS
The body possesses several defense systems. Non-specific defense mechanisms (sometimes called non-specific immunity) fight against a variety of foreign invaders. Listed here are several of the body’s non-specific defense mechanisms:
• The skin provides a physical barrier and secretes enzymes that kill or reduce the virulence of bacteria.
• Mechanical reactions, such as coughing or sneezing, help remove pathogenic material.
• Chemical barriers, such as the normal flora (microorganisms) of the GI system, neutralize or kill microorganisms.
• Tears dilute and wash away irritating substances and microbes.
• Neutrophils, dendritic cells, and monocytes (cellular barriers) ingest and destroy bacteria and toxins and remove cellular debris. They “patrol” the body looking for pathogens, or they can be called in by cytokines. The macrophages live in tissues and produce enzymes and other chemicals. The neutrophils are the most common macrophages. They go to the site of inflammation (by a process called chemotaxis) and are the first to arrive. They are antigen-presenting cells and activate the adaptive immune system, as well as clean out debris from the infection.
• Interferon, a protein made by several types of cells, inhibits virus production and infection.
• Fever and inflammation intensify the effects of interferons, inhibit the growth of some microbes, and speed up body reactions, aiding in tissue repair.
• The respiratory tract contains cilia and macrophages (phagocytic cells) in its mucous membrane lining that trap and remove microbes and dust. Tiny hairs in the nose also serve as a mechanical barrier to dust and other foreign particles.
• The stomach contains hydrochloric acid, which destroys pathogens that might be taken in with foods.
• Other substances, such as earwax, mucus, vaginal secretions, and semen, provide some protection against pathogens.
• Vomiting, defecation, and urination expel microbes from the body, along with the normal waste products.
SPECIFIC DEFENSE MECHANISMS
Specific defense mechanisms (specific immunity) are considered the final line of defense against disease. Specific defense mechanisms are able to recognize and respond to specific foreign substances. Cellular defenses include humoral immunity, which occurs quickly when lymphocytes recognize a foreign substance. Cell-mediated immunity occurs more slowly and depends on T lymphocytes. Both types of immunity are considered specific defense mechanisms because they act against specific, individual harmful substances. Specific immunity can be classified into two main categories: inborn and acquired (Fig. 24-3). Both are based on an antigen-antibody response.
Inborn Immunity
Inborn immunity refers to immunity that is inherited or genetic. This inherited, natural, or innate immunity may be common to all members of a species (e.g., humans have specific immunity to many diseases of animals). Inborn immunity may also be common to a specific population, sex, or ethnic group, or to an individual person.
FIGURE 24-3 · Types of specific immunity
Acquired (Adaptive) Immunity
Acquired or adaptive immunity is attained through natural or artificial sources. Both naturally and artificially acquired immunity can be attained either actively or passively (see Fig. 24-3).
Naturally Acquired Immunity
Naturally acquired immunity occurs when a person is not deliberately exposed to a causative agent. This immunity can occur both actively and passively.
Naturally Acquired Active Immunity. Naturally acquired active immunity results when a child is exposed to and develops a disease (e.g., measles or chickenpox) and subsequently builds up antibodies (immunity) to infections that are caused by the same organism. Individuals can also develop acquired immunity during their lives as they are exposed to disease-causing organisms. They do not necessarily have to become ill with the disease; they build up immunity slowly. In other words, acquired immunity is built on lifetime exposures. Remember that the body manufactures not only cells that target the infecting antigen, but also memory cells. Each time the person is exposed to a disease, the memory cells activate a response that produces antibodies to the offending antigen. Usually, the response is faster with each exposure, as the “memory” increases. Naturally acquired active immunity can last a few years or a lifetime.
Key Concept Exposure to disease-causing organisms during one’s life stimulates the process of acquired immunity Naturally Acquired Passive Immunity. Naturally acquired passive immunity occurs between mothers and their infants. Immunity is transferred from mother to fetus during pregnancy via the placental circulation exchange. If the baby is breast-fed, the baby also receives protection after birth through the mother’s breast milk. Naturally acquired passive immunity can last to 6 months of age, when the infant’s own immune system begins to take over. (The infant does not synthesize the antibodies; they are “borrowed” from the mother.)
Artificially Acquired Immunity
Artificially acquired immunity occurs when a person is deliberately exposed to a causative agent. Artificially acquired immunity can also be acquired through active or passive means.
Artificially Acquired Active Immunity. Artificially acquired active immunity occurs through an injection of the causative agent (antigen) into the person’s system. This is called vaccination, inoculation, or immunization; the substance injected is called a vaccine. The causative agent is diluted to reduce its virulence (strength) so the recipient will form antibodies without becoming ill. (The presence of the antigen causes antibody formation in the person’s body.) Examples of vaccines are those for pertussis (whooping cough), influenza, measles (rubeola), German measles (rubella), and mumps. Many healthcare workers are immunized for hepatitis B as well. Tetanus is an example of an immunization that can be either active or passive. The active form is given as a tetanus booster and causes the person to form his or her own antibodies against tetanus.
Key Concept A vaccine boosts the immune system by offering a weak form of an infection that the body can fight off and can "remember” how to combat when a more virulent form presents itself. Most viral vaccines are given as live attenuated (weakened) viruses. Most bacterial vaccines are based on other components, such as harmless toxins from the organism. Because bacterial vaccines are usually weaker; they often contain other substances designed to induce greater immune response.
Artificially Acquired Passive Immunity. Artificially acquired passive immunity occurs with the injection of ready-made antibodies into a person’s system. These antibodies were produced by another individual’s immune system. An example of this type of immunity is the immunization for rabies. This immunization contains ready-made anti-rabies antibodies and is given in the event of a bite by a rabid animal. This immunization is also usually required if the animal cannot be located and tested. Tetanus toxoid can also be given in the passive form if a person has become ill with the disease of tetanus.
Another type of artificially acquired passive immunity is instituted with the injection of immunoglobulin IgG (gamma globulin). This immunization is given after disease exposure and results in only short-term immunity.
ANTIGEN-ANTIBODY REACTION
Antigen-antibody reactions begin with the B lymphocytes, whose job is to produce humoral immunity. Humoral 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 harmless to the body. The antibody accomplishes this by binding to the antigen, forming an antigen-antibody complex. This binding can be compared to a “lock-and-key” mechanism; an antibody forms in response to only one specific antigen. The patterns on the membrane surface of the antigen and antibody must fit together perfectly. After attaching to the antigen, the antibody uses one of several mechanisms to disarm the antigen. The antibody can neutralize the antigen’s toxins, or the antibody can cause harmful cells to clump together so that macrophages and phagocytes can destroy them. (Antibodies promote or enhance phagocytosis by helping phagocytes attach to the cells they will destroy.)
The complement system attacks the surface of an antibody-coated foreign cell, helping the antibodies to kill the pathogen. The mechanism here is called complement fixation. A complement is a group of proteins normally present, but inactive, in the blood. The complement combines with the antigen-antibody complex and helps in the attack on invading antigens. Complements become active when exposed to the altered cellular shape caused by the antigen-antibody complex. When activated, complements cause the formation of highly specialized antigen-antibody complexes that target specific cells. As previously described, killer T cells release cytotoxins, which cause pores or holes to develop in cell membranes. Sodium and water flow into the cells, causing them to burst open and be destroyed.
Immune system disorders are being identified more frequently today and are believed to be the cause of many other—as yet unidentified—disorders.
BOX 24-2.
Description of the Immune Response
♦ Recognition (of antigen) via antigen processing: Mostly by macrophages. Antigens ingested, broken up, packaged, carried to the surface of cell membrane, and assigned to T-cell receptor
♦ Basophils and eosinophils secrete chemicals to defend against parasites. They also play a role in the allergic reactions in asthma.
♦ Mobilization (of immune system): Cytokines released, other lymphocytes activated, natural killer cells stimulated to secrete interferon. Interleukin-8 acts as signal to guide neutrophils to antigen (chemotaxis).
♦ Attack (killing or eliminating microbes): By macrophages, neutrophils, and natural killer cells. If invading microbe cannot be eliminated, it can be encapsulated or imprisoned by special cells (granuloma); for example, granuloma (tubercle) encloses the bacteria that cause tuberculosis, rendering it unable to cause the illness.
Autoimmune Reaction
♦ Malfunctioning or misinterpretation by immune system of body’s own tissues. (Examples: rheumatoid arthritis, scleroderma, myasthenia gravis, pernicious anemia.)
♦ Mast cells are formed in connective tissue and mucous membranes. They help regulate the inflammatory response (mostly in the situation of allergy and anaphylactic reactions. They contain histamine, heparin, and other substances.
Immunodeficiency Disorders
♦ Immune system is compromised and does not function adequately to prevent infections. Some immune disorders, such as HIV and AIDS, take advantage of the immune system’s weaknesses and make themselves appear to be a normal part of the body.
♦ The innate (biological) immune system (immediate nonspecific reactions), such as phagocytosis and the normal flora (microbes) of the GI system—does not require previous exposure to the antigen—closely related to the nervous and sensory systems
♦ The adaptive (acquired) immune system adapts to specific infection and improves the recognition of the invader (immunological memory humoral and cell-mediated immunity), producing a faster and stronger reaction— requires prior exposure to the antigen—develops to recognize specific pathogens ("immunological memory"), owing to T cells and B cells.
♦ "Herd Immunity" is a term used to describe the phenomenon of organisms living closely together and sharing minor infections all the time.
Some researchers have found that if a person remains calm and unstressed, they are less likely to sustain an autoimmune disorder. This is because the immune system “panics” under stress, believing that something is attacking the body from within.
EFFECTS OF AGING ON THE SYSTEM
Older adults 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 inflammatory process and response to infection. Older adults usually have a baseline body temperature lower than 98.6Έ Therefore, they do not always have a febrile response to infection (Table 24-2). (Fever helps kill microorganisms.)
Special Considerations: LIFESPAN
Infections frequently manifest themselves in older adults as a change in mental status.The cardiovascular system cannot keep up with the increased metabolic demands that an unrecognized infection imposes.The result is cerebral hypoxia or "delirium." Because of this atypical presentation, older adults are susceptible to developing bacteremia (bacteria in the blood) that, if untreated, can quickly progress to septic shock.
NCLEX Alert You should know terminology related to immunity as well as the processes related to the development of immunity Be alert to the needs of the immune system across the lifespan. Nursing actions can involve client safety prevention of infection, and concepts related to client teaching. Appropriate actions can involve multiple concepts that are intertwined in the NCLEX clinical situations.
TABLE 24-2. Effects of Aging on the Immune System
|
FACTOR |
CHANGE |
RESULT |
NURSING IMPLICATIONS |
|
T cells and B cells |
Numbers decrease |
Slowed immune system reaction Increased incidence of tumors Greater susceptibility to infections |
Assess regularly for signs of infection. |
|
Body temperature |
Baseline temperature is lower |
Absence of febrile response to infection |
Observe clients closely for clinical signs or symptoms of infection. |
|
Offer blankets and keep room warm. |
|||
|
Change in temperature is often more significant than actual temperature. |
KEY POINTS
• Immunity is the specific resistance to disease that involves the production of a specific lymphocyte or antibody against a specific antigen.
• Both B cells and T cells derive from stem cells in the bone marrow.
• B cells go on to mature in the bone marrow, whereas T cells complete their maturation and develop immuno-competence in the thymus gland.
• Antigens (antibody generators) are substances (usually proteins) the immune system recognizes as foreign.
• An antibody is a protein that reacts specifically with the antigen that triggers its production.
• Humoral immunity refers to destruction of antigens by antibodies.
• Immunity can be inborn or acquired. Both naturally and artificially acquired immunity can be actively or passively acquired.
• Cell-mediated immunity refers to destruction of antigens by T cells.
• Exposure to disease-causing organisms over one’s lifetime stimulates the process of acquired immunity.
• Humoral or antibody-mediated immunity protects the body against circulating disease-producing antigens and bacteria.
• Antibodies use several mechanisms to destroy antigens: neutralizing toxins, facilitating phagocytosis, imprisoning invader cells (granuloma), and complement fixation.
