Stephanie Tran, Whitney Thompson
Mr. Renfrew
April 4, 2008
Period 3
The Immune System
The immune system is the defense mechanism which our body uses to counter the many foreign objects we come into contact everyday by identifying and destroying pathogens. The immune system is ‘layered’ in order to provide many levels of defense including a physical barrier, a general response, and a specific response.
When a pathogen attempts to invade the body, it must first pass the external layer of skin and mucous membranes. Normally, intact skin cannot be penetrated by bacteria or viruses; however, abrasions may allow passage. The skin itself has a pH of 3 to 5, being acidic enough to kill most potential invading microorganisms. Furthermore, saliva, tears, and mucous secretions further help to wash away bacteria and viruses from the surface of the skin. Lysozymes, enzymes that digest cell walls, acts as a antimicrobial protein by destroying microbes entering the upper respiratory system and the openings around the eyes. The body also expels pathogens by either coughing or sneezing. The trachea also contains specialized epithelial cells equipped with cilia that sweep out microbes and other particles trapped by the mucus, preventing them from entering the lings while microbes in food or in swallowed mucus pass through the acidic gastric juice, destroying most microbes before they are able to enter the intestinal tract. Should a pathogen get past the physical defenses, it must now contend with the internal mechanisms of nonspecific defense. This defense relies mainly on phagocytosis, the ingestion of invading particles by certain types of white blood cells. Neutrophils make up 60%-70% of all white blood cells and can enter infected tissue by amoeboid movement, destroying both the microbe and themselves in that tissue. Monocytes, 5% of all white blood cells, circulate the blood after maturity, migrate into tissue, and then enlarge and develop into macrophages. Macrophages are long-living phagocytes which extend pseudopodia which will pull in microbes that will then be destroyed by digestive enzymes and reactive forms of oxygen. Some macrophages reside in organs and connective tissue; and although they are in a fixed place, they are located where infectious agents circulating the blood and lymph will contact them. Eosinophils comprise of 1.5% of all white blood cells by contain destructive enzymes in their cytoplasmic granules and contribute to defense against larger parasitic invaders by positioning themselves extend the external wall of the parasite and discharging their destructive enzymes. Finally, nonspecific defense also employs natural killer cells, which do not attack the microorganism but the infected cells in the body, which could harbor viruses, and can also destroy injured cells which could from tumors by causing these cells to lyse. The complement system is a group of twenty proteins which act together in activation steps which lead to the lysis of invading microbes. Some parts of the complement system function in chemotaxis to attract phagocytes to sites of infection. The complement system is essential to both the nonspecific and specific defense systems. Interferons are substances that virus-infected cells produce to help other cells interfere with and resist infection by a virus and are most effective in controlling short-term infections. They are secreted by an infected cell as an early and nonspecific defense before specific antibodies appear and will not save the infected cell, but can produce other proteins to inhibit viral replication.
The inflammatory response occurs when there is a physical injury to tissue and is initiated by chemical systems such as histamine, which is contained in basophils in mast cells in connective tissues. Small blood vessels near the injury dilate and become more permeable, increasing blood supply and fluids to the area and causes redness, heat, and swelling. The vasodilation and increase in blood vessel permeability deliver prostaglandins to promote blood flow and clotting elements. Usually an hour after injury, phagocytes migrate to the area through intersistial fluid. Several neutrophils will arrive first and then monocytes which develop into macrophages, devouring pathogens and cleaning up dead tissue and neutrophils. The nonspecific defense does not target specific pathogens such as certain bacteria whereas the specific defense does.
Specific defense is the immune system’s third line of defense. The immune system uses specificity, diversity, memory, and self & nonself recognition to develop a specific response against every foreign microbe, toxin, or transplanted tissue. Specificity is the immune system’s ability to organize and eliminate antigens, particular microorganisms and foreign molecules. Each antigen has a unique molecular shape that will stimulate the production of an antibody by specialized lymphocytes that will attack the antigen. The immune system can respond to millions of invaders with recognizable antigen markers because the immune system has an enormous variety of lymphocyte populations, each with receptors for particular antigens that will be stimulated by a specific antigen and synthesize and secrete the appropriate antibody. Once the immune system has encountered an antigen, it has the ability to remember and react to the same antigen more promptly and effectively if it should encounter it again, an ability known as acquired immunity. Finally, the immune system can distinguish the body’s own molecules from foreign molecules.
The body can gain two kinds of immunity: active immunity and passive immunity. Active immunity depends on the response of the person’s own immune system and can be either naturally acquired, through contracting the illness and recovering, or it can be artificially acquired, through vaccines, which can be inactivated bacterial toxins, killed microorganisms, or weakened microorganism which no longer cause disease but retain their ability to stimulate an immune response. Passive immunity is immunity transferred from one individual to another. It occurs naturally between a mother and an infant and it can also be transferred artificially by introducing antibodies from an animal or human who is already immune to the disease. Passive immunity only lasts for a few weeks or month and the individual must develop its own immunity. The immune system can have two different responses to antigens, humoral and cell-mediated. Humoral immunity causes the production of antibodies cell-mediated immunity depends on the direct action or lymphocytes. The humoral response is normally carried out by the B lymphocytes and the T lymphocytes function in the cell-mediated response. Lymphocytes originate in the pluripotent stem cells in the bone marrow and, depending on where they marture, form B or T lymphocytes. Both B and T cells have specific antigen receptors by the antigen receptors on a B cell are membrane-bound antibodies for a specific antigen whereas the antigen receptors of T cells recognize antigens as specifically as antibodies do. When the antigen binds to the specific receptors of lymphocytes, they activate to divide and differentiate into a population of effector cells which will defend the body in an immune response. B cells will give rise to plasma cells, which secrete antibodies to help eliminate the antigen and T cells give rise to cytotoxic T cells, which kill infected and cancer cells, and helper T cells, which secrete protein factors called cytokines, which are molecules that are regulate neighboring cells. A lymphocyte’s antigenic target specificity
When the immune system fails, the microbes that are able to get past the body's defenses cause a wide array of effects that lower the body's capability of functioning properly. The most common and well known immune failures are allergies. Symptoms of allergies usually occur due to the immune system overreacting to certain substances, such as pollen and animal fur. Some examples of reactions to allergies are itching, eczema, and swelling. Multiple drugs are available to the general population that can temporarily hinder the effects of the offensive substances in order to control the allergies. The failure of the immune system could result in Di George Syndrome, a rare form of primary immunity deficiency which results from the deletion of a part of a chromosome. Some of its symptoms include immune deficiency, lack of T cells, and recurrent infections. Di George Syndrome can come from abnormal fetus development about twelve weeks into pregnancy, interfering with thymus formation and maturation. One solution to this syndrome is intravenous immunoglobulin, which contains the pooled IgG immunoglobulins, antibodies extracted from the plasma of over a thousand blood donors. It has a lasting effect from two weeks to three months, so it is a common solution taken by those with DiGeorge's Syndrome. The failure of the immune system can have a wide spectrum of effects of the body depending on the severity of the harmful substance enters the body.
The releasing walls and preventative measures against the viruses, bacteria, and other harmful pathogens that try to get into the body trigger the immune system. Skin and liquid secretions from the nose and membranes lining the mouth and down are the main stoppers of harmful pathogens from getting into the body. With the skin having a low pH, most microbes burn at the touch; thereby, dying. The mucous coming from the nose and eyes are able to trap and swallow the microbes that try to enter through the holes of the body. However, that does not stop everything, as many still get by the incredible defenses. The second line of defense is phagocytic white blood cells, antimicrobial proteins, and self inflammation. The actual immune system doesn't kick in personally until the pathogen has passed the second line of defense. Responding to an antigen, the immune system releases antibodies to attack the invading pathogen. Antigens attract the antibodies, as they mark the surfaces of foreign materials, like common allergies. With a large diversity, the immune system is able to unleash a large variety of antibodies, dealing with whatever happens to enter the body and is unwanted. The antibodies have the ability to tell what is part of the body and what is harmful to it, therefore allowing it to attack to pathogen. Once the antibodies have reached the harmful pathogen, it is able to work on getting rid of it.