Immunity And Allergy PDF

RESISTANCE OF BODY TO INFECTION IMMUNITY AND ALLERGY  The human body has the ability to resist almost all types of organisms or toxins that tend to damage the tissues and organs. This capability is called immunity. Much of immunity is acquired immunity that does not develop until a...

RESISTANCE OF BODY TO INFECTION IMMUNITY AND ALLERGY  The human body has the ability to resist almost all types of organisms or toxins that tend to damage the tissues and organs. This capability is called immunity. Much of immunity is acquired immunity that does not develop until after the body is first attacked by a bacterium, virus, or toxin, often requiring weeks or months to develop the immunity. An additional portion of immunity results from general processes, rather than from processes directed at specific disease organisms. This is called innate immunity. Innate immunity includes the following 1. Phagocytosis of bacteria and other invaders by white blood cells and cells of the tissue macrophage system. 2. Destruction of swallowed organisms by the acid secretions of the stomach and the digestive enzymes. 3. Resistance of the skin to invasion by organisms. 4. Presence in the blood of certain chemical compounds that attach to foreign organisms or toxins and destroy them. Some of these compounds are (1) lysozyme, a mucolytic polysaccharide that attacks bacteria and causes their degradation. (2) basic polypeptides, which react with and inactivate certain types of grampositive bacteria. (3) the complement complex that is a system of about 20 proteins that can be activated in various ways to destroy bacteria. (4) natural killer lymphocytes that can recognize and destroy foreign cells, tumor cells, and even some infected cells. Acquired (Adaptive) Immunity Acquired immunity is caused by a special immune system that forms antibodies and/or activated lymphocytes that attack and destroy the specific invading organism or toxin. Basic Types of Acquired Immunity—Humoral and Cell-Mediated Humoral immunity or B-cell immunity the body develops circulating antibodies, which are globulin molecules in the blood plasma that are capable of attacking the invading agent. Cell-mediated immunity or T-cell immunity the formation of large numbers of activated T lymphocytes that are specifically crafted in the lymph nodes to destroy the foreign agent.  Because acquired immunity does not occur until after invasion by a foreign organism or toxin, the body must have some mechanism for recognizing the invasion. Each invading organism or toxin usually contains one or more specific chemical compounds that are different from all other compounds; these compounds are called antigens, and they initiate the development of acquired immunity. The Thymus Gland Preprocesses T Lymphocytes (CELL MEDIATED IMMUNITY) Although all lymphocytes in the body originate from lymphocyte-committed stem cells of the embryo, these stem cells themselves are incapable of forming directly either activated T lymphocytes or antibodies. The T lymphocytes, after origination in the bone marrow, first migrate to the thymus gland. Here they divide rapidly and at the same time develop extreme diversity for reacting against different specific antigens. That is, one thymic lymphocyte develops specific reactivity against one antigen. This continues until there are thousands of different types of thymic lymphocytes with specific reactivities against many thousands of different antigens. These different types of preprocessed T lymphocytes now leave the thymus and spread by way of the blood throughout the body to lodge in lymphoid tissue everywhere. The thymus also makes certain that any T lymphocytes leaving the thymus will not react against proteins or other antigens that are present in the body’s own tissues.  The thymus selects which T lymphocytes will be released by first mixing them with virtually all the specific “self-antigens” from the body’s own tissues. If a T lymphocyte reacts, it is destroyed and phagocytized instead of being released. This happens to up to 90 percent of the cells. Thus, the only cells that are finally released are those that are nonreactive against the body’s own antigens—they react only against antigens from an outside source, such as from a bacterium, a toxin, or even transplanted tissue from another person. Once the specific lymphocyte is activated by its antigen, it reproduces wildly, forming tremendous numbers of duplicate lymphocytes. Each set of lymphocytes capable of forming one specific antibody or activated T cell is called a clone of lymphocytes.  Most of the preprocessing of the T lymphocytes in the thymus occurs shortly before and after birth. Removal of the thymus gland after this time diminishes but does not eliminate the T lymphocyte system. Removal of the thymus several months before birth, however, prevents the development of all cell-mediated immunity. Liver and Bone Marrow Preprocess the B Lymphocytes (HUMORAL IMMUNITY) In the human being, B lymphocytes are known to be preprocessed in the liver during mid–fetal life and in the bone marrow during late fetal life and after birth. B lymphocytes differ from T lymphocytes; they actively secrete antibodies, which are large protein molecules capable of combining with and destroying substances. B lymphocytes have a greater diversity than T lymphocytes, forming millions, perhaps even billions, of antibodies with different specific reactivities. After processing, B lymphocytes migrate to lymphoid tissues throughout the body, where they lodge in locations near the T-lymphocyte areas. When a specific antigen comes in contact with the T and B lymphocytes in the lymphoid tissue, a set of T and B lymphocytes becomes activated to form activated T cells and activated B cells, which subsequently form antibodies. The activated T cells and newly formed antibodies react specifically with the antigen that initiated their development and inactivate or destroy the antigen. Humoral Immunity ( B cell )and the Antibodies On entry of a foreign antigen, the macrophages in the lymphoid tissue phagocytize the antigen and present it to adjacent B lymphocytes. The previously dormant B lymphocytes specific for the antigen immediately enlarge and eventually become antibody-secreting plasma cells. The plasma cells produce γ-globulin antibodies, which are secreted into the lymph and carried to the circulating blood.  Some of the B lymphocytes form new B lymphocytes similar to those of the original clone. These B lymphocytes circulate throughout the body and inhabit all the lymphoid tissue but remain immunologically dormant until activated again by a new quantity of the same antigen. These are called memory cells. Subsequent exposure to the same antigen causes a more rapid and potent antibody response because of the increased number of lymphocytes in the specific clone. Antibodies Are γ-Globulin Proteins Called Immunoglobulins There are five general classes of antibodies, each with a specific function: IgM, IgA, IgG, IgD, and IgE. The IgG class is the largest and constitutes about 75% of the antibodies of a normal person.  The antibodies can inactivate the invading agent directly in one of the following ways: Agglutination, in which multiple large particles with antigens on their surfaces, such as bacteria or red blood cells, are bound together in a clump. Precipitation, in which the molecular complex of soluble antigens and antibodies becomes so large that it is rendered insoluble. Neutralization, in which the antibodies cover the toxic sites of the antigenic agent. Lysis, in which antibodies are occasionally capable of causing rupture of an invading cell by directly attacking the cell membranes. Although antibodies have some direct effects in the destruction of the invaders, most of the protection afforded by antibodies derives from the amplifying effects of the complement system.  The Complement System Is Activated by the Antigen-Antibody Reaction Complement is a collective term used to describe a system of proteins normally present in the plasma that can be activated by the antigen antibody reaction. When an antibody binds with an antigen, a specific reactive site of the antibody becomes uncovered, or activated. This activated antibody site binds directly with the C1 molecule of the complement system, setting in motion a cascade of sequential reactions. When complement is activated, multiple end products are formed. Several of these end products help destroy the invading organism or neutralize a toxin. Complement can stimulate 1.Phagocytosis by both neutrophils and macrophages. 2.Cause rupture of the cell membranes of bacteria or other invading organisms 3.Promote agglutination 4.Attack the structure of viruses 5.Promote chemotaxis of neutrophils and macrophages 6.Induce the release of histamine by mast cells and basophils, promoting vasodilation and leakage of plasma, which in turn promote the inflammatory process. Activation of complement by an antigen antibody reaction is called the classical pathway.  Activated T Cells and Cell-Mediated Immunity When macrophages present a specific antigen, T lymphocytes of the specific lymphoid clone proliferate, causing large numbers of activated T cells to be released. These activated T cells pass into the circulation and are distributed throughout the body, where they circulate for months or even years. T-lymphocyte memory cells are formed in the same way that B memory cells are formed in the antibody system. Antigens bind with receptor molecules on the surface of the T cells in the same way they bind with antibodies.  B lymphocytes recognize intact antigens. T lymphocytes respond to antigens only when they are bound to specific molecules called MHC proteins on the surface of an antigen presenting cell. There are three major types of antigen-presenting cells: macrophages, B lymphocytes, and dendritic cells. Dendritic cells are located throughout the body and are most effective in presenting antigens to T cells. The three major groups of T cells are helper T cells, cytotoxic T cells, and suppressor T cells 1.Helper T cells serve as regulators of virtually all immune functions, through the formation of a series of protein mediators called lymphokines, which act on other cells of the immune system and bone marrow. Helper T cells secrete interleukins 2-3-4-5-6, granulocyte-monocyte colony stimulating factor, and interferon-γ. In the absence of the lymphokines produced by the helper T cells, the remainder of the immune system is almost paralyzed. It is the helper T cells that are inactivated or destroyed by the human immunodeficiency virus (acquired immunodeficiency syndrome), which leaves the body almost totally unprotected against infectious disease  Helper T cells perform the following functions: Stimulate growth and proliferation of cytotoxic and suppressor T cells through the actions of interleukins 2, 4, and 5. Stimulate B cell growth and differentiation to form plasma cells and antibodies mainly through the actions of interleukins 4, 5, and 6. Activate the macrophage system. Stimulate helper T cells themselves. Interleukin-2 has a direct positive feedback effect of stimulating activation of the helper T cell, which acts as an amplifier to enhance the cellular immune response  Cytotoxic T Cells are Capable of Killing Microorganisms through a Direct Attack For this reason, they are also called killer cells. Surface receptors on the cytotoxic T cells cause them to bind tightly to those organisms or cells that contain their binding-specific antigen. After binding, the cytotoxic T cells secrete hole-forming proteins, called perforins, which literally punch large holes in the membrane of the attacked cells. These holes disrupt the osmotic equilibrium of the cells, leading to cell death. Cytotoxic T cells are especially important for destroying cells infected by viruses, cancer cells, or transplanted organ cells.  Suppressor T Cells Suppress the Functions of both Cytotoxic and Helper T Cells. It is believed that these suppressor functions serve the purpose of regulating the activities of the other cells so excessive immune reactions that might severely damage the body do not occur.  The immune system normally recognizes a person’s own tissue as being completely distinct from that of invading organisms. It is believed that most of the phenomenon of tolerance develops during the processing of T lymphocytes in the thymus and B lymphocytes in the bone marrow. It is thought that continuous exposure to self-antigen in the fetus causes the self-reacting T and B lymphocytes to be destroyed. Failure of the tolerance mechanism leads to autoimmune diseases in which the immune system attacks the tissues of the body, such as rheumatic fever, in which the body becomes immunized against the tissues of the joints and valves of the heart. Immunization by Injection of Antigens Active immunity A person can be immunized by injecting dead organisms that are no longer capable of causing disease but that still have some of their chemical antigens. Immunity can be achieved against toxins that have been treated with chemicals so that their toxic nature has been destroyed even though their antigens for causing immunity are still intact. finally, a person can be immunized by being infected with live organisms that have been “attenuated.” That is, these organisms either have been grown in special culture media or have been passed through a series of animals until they have mutated enough that they will not cause disease but do still carry specific antigens required for immunization.  Passive Immunity This is done by infusing antibodies, activated T cells, or both obtained from the blood of someone else or from some other animal that has been actively immunized against the antigen. Antibodies last in the body of the recipient for 2 to 3 weeks, and during that time, the person is protected against the invading disease. Activated T cells last for a few weeks if transfused from another person but only for a few hours to a few days if transfused from an animal. Allergy and Hypersensitivity An important but undesirable side effect of immunity is the development of allergy or other types of immune hypersensitivity. Allergy can be caused by activated T cells and can cause skin eruptions, edema, or asthmatic attacks in response to certain chemicals or drugs. Some allergies are caused by IgE antibodies. A special characteristic of IgE antibodies is their ability to bind strongly with mast cells and basophils, causing the release of multiple substances that induce vasodilation, increased capillary permeability, and attraction of neutrophils and eosinophils.  Hive ( also known as urticaria— is a skin reaction that causes itchy welts that range in size from small spots to large blotches).  Hay fever (allergic rhinitis) is an allergic response from your immune system that causes sneezing, runny nose and watery, itchy eyes. Hay fever can be triggered by seasonal allergens like pollens and grass or year-round triggers like dust mites, and animal fur.