An Excellent Study Guide for The Human Immune System

The immune system of the human body is a very unique and intricate work of art. This bullet point study guide highlights all of the key components of a college level study on the human immune system.

  • Phagocytosis is the engulfing and destroying of foreign substances by forming a vacuole that fuses with a lysosome.
  • Macrophages are large phagocytic cells that wander through the interstitial fluid and “eat” bacteria and virus-infected cells that they encounter.
  • Natural killer cells are not phagocytes. They attack cancer cells and virus infected cells by releasing chemicals that promote programmed cell death.
  • Interferons are proteins produced by virus-infected cells that help other cells resist viruses.
  • The complement system is a group of about 30 different proteins that circulate in an inactive form in the blood and can act together with other defense mechanisms.
  • The inflammatory response: 1) Damaged cells soon release chemical alarm signals such as histamine. 2) The chemicals spark the mobilization of various defenses. Histamine induces neighboring blood vessels to dilate and become leakier allowing for plasma and white blood cells to leak into the area. 3) The white blood cells then engulf the bacteria in the area. Many white blood cells themselves die during this process and are consumed; this is where pus often comes from.
  • Sometimes bacterial infections bring about an overwhelming systematic inflammatory response leading to a condition called septic shock; characterized by high fever and low blood pressure.
  • The lymphatic system is involved in both innate and acquired immunity and has two main functions: to return tissue fluid to the circulatory system and to fight infection.
  • The lymphatic vessels carry fluid called lymph which is similar to interstitial fluid but contains less oxygen and fewer nutrients.
  • When lymph infected with bacteria enters lymphatic organs, microphages permanently residing there may engulf the bacteria as a part of the innate immune response.
  • When your body is fighting infections such as mononucleosis, the organs of the lymphatic system become a major battle ground. Lymph nodes fill with huge numbers of defensive cells, causing the tender “swollen glands” in your neck and armpits.
  • Acquired immunity, found only in vertebrates, is a set of defenses that are activated only after exposures to pathogens. Once activated, it provides a strong defense against pathogens that is highly specific. Moreover, acquired immunity can amplify certain innate responses, such as inflammation and the complement system.
  • Any foreign molecule that elicits an acquired immune response is called an antigen.
  • When the immune system detects an antigen, it responds with an increase in the number of cells that either attack the invader directly or produce immune proteins called antibodies.
  • An antibody is a protein found in the blood plasma that attaches to one particular kind of antigen and helps counter its affects.
  • Acquired immunity has a remarkable memory and it can react much more quickly to an antigen the second time it is encountered.
  • Acquired immunity is usually obtained by natural exposure to antigens, but it can also be achieved by vaccination.
  • In this procedure, the immune system is confronted with a vaccine composed of a harmless variant of the disease-causing microbe. The vaccine stimulates the immune system to mount defenses against this harmless antigen that will be used when the system faces the actual harmful pathogen because the two are much alike.
  • Whether antigens enter the body naturally or artificially, the resulting immunity is called active immunity because the person’s own immune system actively produces antibodies.
  • It is also possible to acquire passive immunity by receiving premade antibodies. Passive immunity is temporary because the recipient’s immune system is not stimulated by the pathogens and the immunity only lasts as long as it takes the body to break down the injected antibodies.
  • Lymphocytes, white blood cells that spend most of their time in the tissues and organs of the lymphatic system, are responsible for the acquired immune response.
  • The humoral immune response involves the secretion of antibodies by B cells into the blood and lymph. This system primarily defends against bacteria and virus present in body fluids. It can also be passed passively by injecting the blood plasma from an immune individual into a non-immune individual.
  • The second type of acquired immunity, produced by T cells, is called the cell-mediated immune response. This defensive system results from the action of defensive cells (rather than the defensive proteins of the humoral response). Certain T cells attack body cells that are infected with bacteria or viruses. Other T cells function indirectly by promoting phagocytosis by other white blood cells and by stimulating B cells to produce antibodies.
  • Every T or B cell has molecules on the exterior, antigen receptors, capable of binding one specific type of antigen. Each T or B cell has about 100,000 antigen receptors, and all of these receptors on a single cell are identical – they all recognize the same antigen.
  • Because lymphatic capillaries extend to virtually all of the body’s tissues, bacteria or viruses infecting nearly any part of the body eventually enter the lymph and are carried to the lymphatic organs.
  • An antibody usually recognizes and binds to a small surface-exposed region of an antigen, called antigenic determinant (also known as epitope).
  • An antigen-binding site, a specific region on the antibody molecule, recognizes an antigenic determinant by the fact that the binding site and antigenic determinant have complimentary shapes.
  • An antigen generally has multiple determinants so it may stimulate the immune system to produce ­several distinct antibodies against it.
  • The immune system’s ability to defend against an enormous variety of antigens depends on a process known as clonal selection.
  • Once inside the body an antigen encounters a diverse pool of T and B cells but only interacts with a tiny fraction of the cells depending on the receptors specific to that antigen.
  • Once activated by this antigen, these few T or B cells ­proliferate, forming a clone of thousands of cells to fight that specific antigen.
  • This antigen-driven cloning of lymphocytes, clonal selection, is a vital step in the acquired immune response against infection.
  • Steps of clonal selection: 1) There are multiple T and B cells in the lymph node and when an antigen enters it stimulates a specific T or B cell. 2) The selected cell is activated and grows, divides, and differentiates into two genetically identical yet physically distinct type of cells. 3) The first group of newly cloned cells are effector cells, which combat the antigen. Because the antigen triggered a B cell in this example the effector cells are plasma cells. Each plasma cell makes as many as 2,000 copies of its antibody per second. These antibodies circulate in the blood and lymphatic fluid, contributing to the humoral immune system. Each effector cell only last 4-5 days. 4) The second set of cells produced by the activated B cell are a smaller number of memory cells. These cells remain in the lymph nodes and can live for decades poised to fight a second encounter with the antigen. Steps 1-4 show the initial phase of acquired immunity, called the primary immune response. 5) When memory cells produced during the primary response are activated by a second exposure to the same antigen, they initiate the secondary immune response. This response is faster and stronger than the first.
  • B cells are the “frontline warriors” of the humoral immune response.
  • Each antibody molecule is made up of four polypeptide chains, two identical “heavy chains” and two identical “light” chains.
  • An antibody molecule has two related functions in the humoral immune response: to recognize and bind to a certain antigen, and to assist in neutralizing the antigen it recognizes.
  • An antigen-binding site is a region of the molecule responsible for the antibody’s recognition-and-binding function.
  • Humans and other mammals have five major classes of antibodies found in different parts of the body but they all serve the same purpose; to mark invaders for elimination.
  • An antibody marks an antigen by combining to form an antigen-antibody complex.
  • It is the binding of antibodies to antigens that actually triggers mechanisms to neutralize or destroy an invader. Such a mechanism is called an effector mechanism.
  • There are four main effector mechanisms:
    • Viral neutralization is where antibodies bind to the surface proteins of a virus, thereby blocking its ability to infect a host cell. The bound antibodies enhance macrophage destruction of the virus/bacteria.
    • Agglutination (clumping together) of viruses, bacteria, or foreign eukaryotic cells is another mechanism. Because antibodies have two binding sites they can clump the cells together making it easier for phagocytes to capture them.
    • A third effector mechanism is precipitation. The antibody molecules link dissolved antigen molecules together. This also enhances engulfment by phagocytes.
    • Possibly the most important effector mechanism is activation of complement system by antigen-antibody complexes. Activated complement system proteins can attach to foreign cells and poke holes in their plasma membrane, causing a cell to lysis (rupture).
  • The antibodies of the humoral immune response, which identify and bind to foreign invaders, work with innate defense, such as phagocytes and complement, to form a complete defense system.
  • Monoclonal antibodies are powerful in the lab and clinic because of their ability to tag specific molecules or cells. They are widely used in laboratory research, clinical diagnosis, and treatment of disease.
  • In order to make monoclonal antibodies a scientist must fuse the cells from an animal with the desired antibodies to the cells of a rapidly growing cancer cells in a laboratory dish so a large amount of the identical antibody molecules can be made.
  • One common use of monoclonal antibodies is in at home pregnancy tests. They also have great promise in helping fighting diseases such as cancer.
  • One monoclonal antibody, Herceptin, is used to treat a common form of aggressive breast cancer. The drug attaches to the growth factor receptors in the cancer and slows down the communication to other cells delaying growth.
  • Toxin linked antibodies carry out precise search-and-destroy mission, selectively attaching to and killing tumor cells.
  • The humoral defense system identifies and helps destroy invaders outside of our body cells but it is the cell-mediated immune response produce by T cells that battles pathogens that have already entered body cells.
  • T cells only respond to antigens present on the surfaces of the body’s own cells.
  • There are two main kinds of T cells. Cytotoxic T Cells attack body cells that are infected with pathogens. Helper T Cells play a role in many aspects of immunity; they help activate cytotoxic T cells and macrophages and they even help stimulate B cells to produce anitibodies.
  • Helper T cells interact with other white blood cells (macrophages, B cells, and other types of immune cells) that function as antigen-presenting cells.
  • All of the cell-mediated immune response and much of the humoral immune response depend on the precise interaction of antigen-presenting cells and helper T cells. This interaction activates the helper T cells, which can then go on to activate other cells of the immune system.
  • An antigen-presenting cell presents a foreign antigen to a helper T cell (the protein binding is called a self-protein while the antigen is called a nonself molecule. They form a self-nonself complex). Depending on the proteins and antigen being presented the helper T cell with replicate itself and also make memory cells. Then depending on the proteins B cells and cytotoxic T cells can be produced (pg. 497).
  • Helper T cells activate cytotoxic T cells, the only T cells that actually kill infected cells.
  • The cytotoxic T cell and infected body cell from a self-nonself complex. This tell the cytotoxic T cell to begin synthesizing several new proteins, one being perforin. Perforin is discharged and attaches to the infected cell’s membrane, making holes in it. T cell enzymes then enter the infected cell and promote death by apoptosis. The infected cell dies and is destroyed.
  • AIDS (acquired immunodeficiency syndrome) results from an infection by HIV, the human immunodeficiency virus.
  • The virus is transmitted via body fluids carrying infected cells. Most often, HIV enters the body through small cuts or sores during sexual contact or by needles contaminated with infected blood.
  • HIV most commonly attacks helper T cells which severely impairs the processes of the cell-mediated and humoral responses. This drastically compromises the body’s ability to fight infections.
  • HIV attaches to proteins on the surface of a helper T cell. The HIV then enters the cell and begins to reproduce. The RNA genome of HIV is then reverse-transcribed, and the newly produced DNA is integrated into the T cell’s genome. This viral genome can now direct the production of new viruses. Eventually the helper T cell dies from the damaging effects of the virus or virus-triggered apoptosis (programmed cell death). The new HIV released into the bloodstream slowly kills off other helper T cells weakening the body’s defenses against the smallest infections.
  • Full blown AIDS syndrome can take up to 10 years to fully develop after HIV infection.
  • Immune system impairment makes AIDS patients susceptible to opportunistic infections. This means infections and virus that a healthy immune system would resist can be life threatening in AIDS patients.
  • Until there is a vaccine or cure for AIDS the best way to prevent it is practicing safe sex and avoiding intravenous drug use.
  • HIV has one of the fastest rates of mutation of any pathogen ever studied making it very difficult to create AIDS treatment. Current HIV treatment is only a temporary fix and not a cure for aids.
  • Each person’s cells have a particular collection of proteins that provide the molecular “fingerprints” recognized by the immune system. Each of us has two sets of proteins. Class I proteins occur on almost all nucleated cells in the body. Class II proteins are found only on a few types of cells, including B cells, activated T cells, and macrophages. This tells the immune system these cells are “off limits”
  • If lymphocytes with receptors that bind to the body’s own molecules are created they are destroyed or deactivated.
  • The group of self-protein genes is called the major histocompatibility complex, or MHC. It is virtually impossible for two humans to have the same sets of self proteins, except for identical twins. This can be a bad thing at times because organ transplant patients’ bodies can reject the organ. This causes doctors to look for people with the closest matching MHC when doing organ transplants.
  • Malfunction or failure of the immune system causes disease.
  • Autoimmune disease occurs when the immune system goes awry and turns against some of the body’s own molecules. Some examples of this are lupus, rheumatoid arthritis, insulin-dependent diabetes, and multiple sclerosis. Current medicines for treating autoimmune diseases either suppress immunity in general or are limited to alleviation of specific symptoms.
  • There are also immunodeficiency diseases. Immunodeficient people lack one or more of the components of the immune system. This makes them susceptible to frequent and recurrent infections. Immunodeficiency is not always an inborn condition; it may be acquired later in life (AIDS).
  • There is growing evidence that physical and emotional stress can harm immunity.
  • Allergies are hypersensitive (exaggerated) responses to antigens in our surrounding. Antigens that cause allergies are called allergens.
  • The symptoms of an allergy result from a two-stage reaction sequence:
    • The first stage, called sensitization, occurs when a person is first exposed to an allergen. An allergen enters the bloodstream then attaches to a B cell with complimentary receptors. The B cells proliferate through clonal selection and secrete large amount of antibodies to the allergen. Some of these antibodies attach to mast cells, body cells that produce histamine and other chemicals that trigger inflammation.
    • The second stage occurs when a person is exposed to the same allergen later. The allergen binds to the antibodies attached to mast cells which trigger the allergic symptoms.
  • Antihistamines are drugs that interfere with histamine’s action and give temporary relief from an allergy.
  • Anaphylactic shock is an especially dangerous type of allergic reaction. If someone is extremely allergic to an allergen when they come in contact with this allergen it causes their mast cells to release inflammatory chemicals very suddenly causing their blood vessels to dilate abruptly, creating a potentially fatal drop in blood pressure. It can be counteracted with injections of epinephrine.
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One Response to “An Excellent Study Guide for The Human Immune System”
  1. jack peterson Says...

    On October 15, 2010 at 3:23 am

    This post is very informative and useful.. Thanks for sharing knowledge..
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