| Contents | Previous | Next |
The first line of defense of the body’s immune system consists of the skin, the mucous membranes, and the stomach. The second line of defense involves phagocytosis. Pathogenic bacteria can become lethal to humans; for example, a mere thousand grams of botulin bacterium toxin could destroy the entire human race. When antigens successfully invade the body through the skin or mucous membranes, their presence elicits the inflammatory response, which prevents antigenic spread.
We have seen that the immune system is composed of lymphatic vessels, thymus, spleen, liver, tonsils, adenoids, lymph nodes, white blood cells (lymphocytes, neutrophils, basophils, eosinophils, monocytes, etc.) and specialized cells-macrophages or mast cells that reside in tissues. Dr. J.A. Bellanti, author of Immunology, defines immunity as follows. “Immunity may be viewed as an adaptive system in which the body attempts to maintain homeostasis between the internal body environment and the external environment.”
Immune reactions destroy, immobilize, or neutralize disease-causing agents, foreign matter, and certain altered body cells such as tumor cells and autoimmune events. When functioning well, the finely tuned immune system mobilizes cells and antibodies appropriate and specific to the antigen challenge.
Therefore, immunity is the ability of the human body to tolerate the presence of material indigenous to body (“self”) and to eliminate foreign (“non-self”) material. This complex and discriminatory ability provides protection from infectious diseases, since most microbes are identified as foreign by the immune system.
The immune system checks cells and identifies them as “self” or “non-self”. Autoimmune diseases are caused by the immune system attacking itself, and destroying cells and tissues. For unknown reasons cells and molecules sometimes stimulate the adaptive immune responses and the body’s cells are attacked as if they were foreign. This is known as autoimmune disease, or autoimmunity.
Among common diseases caused by immune system errors are: Juvenile-onset diabetes – caused by the immune system attacking and eliminating the cells in the pancreas that produce insulin; Rheumatoid arthritis - caused by the immune system attacking tissues inside the joints. Some evidence also supports Type I diabetes as an autoimmune disease. This is where the T-lymphocytes respond to an antigen that might be located in a joint.
The by-products of the immune reaction inflame the synovia, which causes the common symptoms of stiffness and soreness. Toxicity gradually eats away at the cartilage and joints. Myasthenia gravis (MG) is a muscle weakness caused by destruction of muscle-nerve connections.
Antibodies attacking the myelin of nerve cells cause Multiple Sclerosis (MS). Systemic lupus erythematosis (SLE) has the person forming a series of antibodies to their own tissues, such as kidneys (the leading cause of death in SLE patients) and the DNA in their own cellular nuclei.
Another example of an autoimmune response is the effect of the immune system on transplanted tissue. When the foreign tissue is placed inside the body its cells do not contain the correct identification. The immune system therefore attacks the tissue. The problem cannot be prevented, but can be diminished by carefully matching the tissue donor with the recipient and by using immunosuppressant drugs to try to prevent an immune system reaction. Of course, by suppressing the immune system these drugs open the patient to opportunistic infection.
Extremely important to the immune system is the respiratory tract where allergies caused by airborne particles can set off a false alarm. Thus asthma is also considered to be another autoimmune disease.
Allergies are yet another form of immune system error. For some reason, in people with allergies, the immune system strongly reacts to an allergen that should be ignored. The allergen might be a certain food, or a certain type of pollen, or a certain type of animal fur. It is clear, then, that the immune system can overreact, causing autoimmune diseases
Allergies result from immune system hypersensitivity to antigens that do not cause an immune response in most people. Allergens, substances that cause allergies, include dust, molds, pollen, cat dander, certain foods, and some medicines (e.g., penicillin). Up to 10% of the U.S. population suffers from at least one allergy.
Subsequent exposure to the same allergen causes a massive secondary immune response that releases IgE antibodies. These bind to mast cells found usually in connective tissues surrounding blood vessels. Mast cells then release histamine, which starts the inflammatory response. In some individuals the histamine release causes life-threatening anaphylaxis or anaphylactic shock.
The most common types of allergic reactions-hay fever, some kinds of asthma, and hives-are produced when the immune system responds to a false alarm. In a susceptible person, a normally harmless substance — for example, grasses, pollen or house dust — is perceived as a threat and is attacked.
Such allergic reactions are related to the antibody known as immunoglobulin E. Like other antibodies, each IgE antibody is specific; one reacts against oak pollen, another against ragweed. The role of IgE in the natural order is not known, although some scientists suspect that it developed as a defense against infection by parasitic worms. The first time an allergy-prone person is exposed to an allergen, he or she makes large amounts of the corresponding IgE antibody. These IgE molecules attach to the surfaces of mast cells (in tissue) or basophils (in the circulation). Mast cells are plentiful in the lungs, skin, tongue, and linings of the nose and intestinal tract.
Immunity has the ability to “remember” a foreign substance previously encountered and react again, promptly. There are two kinds of immunity: active immunity, when the body is stimulated to produce its own antibodies, and passive immunity, where the antibodies come from outside the person’s body.
Active immunity, (also known as: adaptive immunity, acquired immunity) is usually permanent, and can be induced due to actual illness or vaccination. Passive immunity is not permanent because the antibodies are introduced from outside the body, thus the B-cells never “learn” how to make them.
The body’s systemic defenses, by themselves, may not be sufficient to protect the host against pathogens. However, even if pathogens evade or overcome the relatively nonspecific constitutive defenses, they may yet be detected and attacked by the more specific inducible defenses, once they have developed.
Thus active immunity or acquired immunity is established when the body is exposed to various antigens and builds a defense that is specific to that antigen. In the case of active immunity the host undergoes an immunological response and produces the cells and factors responsible for the immunity, i.e., the host produces its own antibodies and/or immune-reactive lymphocytes. Active immunity can persist for a long time in the host, up to many years in humans. For example, an individual who has had chickenpox is immune from having chickenpox again.
The active immune system can be divided into two major sections:
Both responses are similar in identifying antigens as foreign. They also both recognize human antigens not made by the individual resulting in graft rejection.
In the humoral type of immunity the lymphocytes act as antibody producing cells, called plasma cells. An antigen and its specific antibody are bound together by physical and electrical forces to neutralize the harmful effect of the antigen. This reaction is called complement and might involve groups working together.
Each B lymphocyte produces a distinct antibody molecule (immunoglobulin or Ig). Since more than a million different B-lymphocytes are produced in each individual it means that each individual can recognize over a million different antigens. Some B-cells become memory cells to produce antibody at a low rate for a long time (long term immunity) and to respond quickly when the antigen is encountered again.
When a new antigen comes into the body it binds to the B-cell which makes an antibody matching the antigen displayed on the cell surface by a special receptor protein (MHC II) for recognition by helper T-cells. The helper T-cell divides and produces secreted antibodies, which circulate in the serum and lymph, activating the B-cell. The response is regulated by a class of T-cells called suppressor T-cells.
Cell mediated immunity (CMI) is a type of response in which antibody production or activity is of minor importance. In cell mediated immunity (sometimes referred as “delayed hypersensitivity”) the lymphoid cells are already programmed to respond as a result of an earlier antigenic stimulation. The antigens in contact with some of these cells trigger the release of an “inflammatory hormone” which attracts other cells. This reaction usually takes a few days to develop.
Extracellular factors that affect immunity include the lymphokines, which are proteins produced by T-lymphocytes that have effects on the differentiation, proliferation and activity of various cells involved in the expression of Cell mediated immunity (CMI).
In general, lymphokines function by:
CMI responses are generated during almost all infections, but the relative magnitude and importance of each type of response shows great variation in different hosts and with different infectious agents.
In some types of infections antibody plays a major role in immunity or recovery. CMI is of supreme importance in recovery with infections where the microbe grows or multiplies intra-cellularly, such as tuberculosis, brucellosis and syphilis.
The clearest picture of the importance of CMI in recovery from disease is seen in certain viral infections (e.g. herpes, pox viruses and measles virus infections). Viruses are intracellular parasites. The best strategic defense against virus-infected cells seems to be to kill the infected cell when the virus may be in a noninfectious- form. Many viruses, as they mature, cause foreign (viral) antigens to appear on the infected cell surface. The infected cell can be destroyed before the virus is liberated.
Passive immunity involves antibodies that are produced outside the body. For example, infants have passive immunity because they are born with antibodies that are transferred through the placenta from the mother. Passive immunity is protection by products produced by an animal or human, then transferred to another human, usually by injection, i.e., the host receives antibodies and/or immune-reactive lymphocytes originally produced during an active response in another animal.
Passive immunity wanes with time, usually a few weeks or months. In either case, active or passive immunity, resistance may be acquired by natural means or by artificial means (i.e., vaccination and immunization procedures). Gamma globulin is another form of passive immunity that is administered by a doctor. Its protection is also temporary.
Antigens originate from within the body or arrive via the outside environment. The immune system provides two lines of defense against them: nonspecific and specific immunities. A first-time encounter with an antigen elicits a nonspecific immune response. Defense mechanisms include skin, mucous membranes, chemicals, specialized cells, and the inflammatory response. For example, a spider bite triggers a nonspecific immune response. White blood cells arrive first on the scene to rid the body of venom antigens.
If the body detects an antigen that it has previously met, a specific immune response occurs. In this response, the body has been trained to recognize and neutralize a familiar specific antigen: its immune system “remembers” the antigen.
Such systemic immunity enables a faster, longer lasting immune response than does a nonspecific response.
To summarize: Immunology is the study of the protection from foreign organisms and responses to foreign organisms, such as viruses, bacteria, protozoa or larger parasites. Immunity to these organisms is defined as an adaptive system in which the body attempts to maintain homeostasis between the internal body environment and the external environment. In addition, immune responses may develop against the body’s own proteins in autoimmunity and against its own aberrant cells in tumor immunity.
The immune system recognizes and destroys anything foreign to the body, including bacteria and foreign particles that include toxic compounds. Cells in the circulatory and the lymphatic systems perform this recognition and destruction.
These cells are produced in the bone marrow and lymphatic tissue (thymus, lymph nodes, spleen and tonsils) respectively. The cells begin their lives as “stem cells.” They are then released into the blood stream and are carried to all parts of the body.
T-cells and B-cells police the body for ‘non-self’ material. T- cell lymphocytes help B-cell lymphocytes kill infected cells. Other opportunistic infections can flourish when the T-cell count dips below 200. When the T-cells drop, secondary infections like malaria and trypanosomiasis (sleeping sickness) can occur. Sometimes, a cell slips by the normal control factors and begins a course of uncontrolled division, (cancer). In the case of cancer all defense mechanisms become activated — interferon, antibodies, complements, and NK cells — in an effort to destroy a tumor.
Interleukin-I is a byproduct of macrophages and this signals a rise in the T-cell production. In turn, the T-cells produce Interleukin-II that leads to B-cell production of antibodies. T-cells are most responsible for detoxification of abnormal cells and they also recognize antigens of potential invader cells.
The immune system produces antibodies and cell-mediated responses (CMI) in which specific cells recognize foreign pathogens and destroy them. Macrophages initiate T-cell interactions, development and proliferation. After an immunological encounter, these cells are activated to produce and/or respond to various classes of lymphokines, mediators of CMI.
The inducible defenses are so-called because they are induced upon primary exposure to a pathogen or one of its products. The inducible defenses are a function of the immunological system and the immune responses. They must be triggered in a host and initially take time to develop. The type of resistance thus developed in the host is called acquired immunity.
Sometimes the immune system makes a so-called error, or mistake. This is known as autoimmunity, where the immune system attacks itself in the same way it would attack an antigen. Why this occurs is still a mystery. In addition, immune responses against the body’s own proteins may develop in autoimmunity and against the body’s own aberrant cells in tumor immunity.