Viral Attack! Your Clean-Up Crew to the Rescue! A Gallery Demonstration at Arizona Science Center Table of Contents Background Information Links Background Information What is the immune system? Our body is constantly on alert for foreign invaders that may threaten its health. The immune system protects the body from sickness. The main function of the immune system is to be able to deal with foreign invaders, whether these are particles or living organisms. Hence the key to the immune system s successful functioning is its ability to distinguish between self and non-self. Once a foreign agent is identified, the body is able to mount a response to it. The immune system is divided into two parts, the Innate Immune System and the Acquired Immune System (also called Adaptive Immune System)). While each of these plays a role in defending the body, there are major differences between the two. The innate immune system, which is our body s built-in immune system and first line of defense, is continually working around the clock to protect the body. Our bodies have natural physical protectors (i.e. skin, mucous membranes, saliva, etc.) as well as an army of cells, called phagocytes (phago = eater; cyto = cell), in our blood whose job it is to hunt down and eat unwanted invaders through a process called phagocytosis. The acquired immune system, on the other hand, is the body s second line of defense and needs to be primed before it can work to its full effectiveness. This system is only fully effective after it has detected a possible infective agent before so it is able to detect it again in the future and prevent it from infecting the body. This system, which operates via our lymphatic system, is responsible for the destruction of foreign particles once they have entered the body. When introduced to an unrecognized invader (virus, bacteria, etc.) the body must learn how to attack and destroy the foreign particle. The main cells involved in this immune response are T-cells and B-cells. Here, while both systems are called into play in Viral Attack!, we will focus primarily on the acquired immune system. What cells are involved in acquired immune response? The activation of the acquired immune system initially requires the help of other cells involved in the body s innate (built-in) response. The cells of the acquired immune system are dotted with receptors, which are designed to recognize certain substances (there are many immune system cells in the blood, each with its own different receptor. This means that the body can be protected against many different things). All agents foreign to your body have unique patterns on their surfaces that allow the cells of the acquired immune system to detect them as foreign.
When the cells of the acquired immune system detect these patterns, the agents are recognized as foreign, and the immune system can initiate an attack through the production of an antibody. Anything that the immune system can detect is called an antigen. An antigen is a substance that, when introduced into the body, induces an immune response consisting of the production of a circulating antibody. An antibody is a molecule that is responsible for recognizing and marking an antigen for destruction by white blood cells. Two broad classes of white blood cells are lymphocytes and macrophages. Two important types of lymphocytes are B-cells and T-cells. The B-cells (which produce antibodies), and T-cells, and macrophages make up the immune response. Other important cells include dendritic cells. B-cells: B-cells perform immune surveillance, make antibodies against antigens, and tag pathogens with antibodies to make them targets for phagocytes (macrophages and dendritic cells). B-cells eventually develop into B memory cells to help the body recognize returning invading microbes. B-cells are named as such because they originate in the bone marrow. T-cells: T-cells are named as such because they develop in the thymus gland. There are two main types of T-cells: Helper T-cells: Their primary task is to activate B-cells and killer T-cells Killer T-cells: Attack cells in the body that have been infected by viruses and/or bacteria and kill them. Macrophages ( big eaters ): These are large phagocyte scavenger cells that engulf and digest foreign agents. They also alert helper T-cells to the presence of foreign invaders. Dendritic cells: These are phagocytes that devour intruders as well as help activate the helper T-cells (immune system). They can also filter bodily fluids to clear them of foreign organisms and particles. Epithelial cells: Cells which line the inner and outer surfaces of the body and are subject to invasion by pathogens. There are two specific pathways of defenses for acquired immunity: Humoral Immunity (B-cell Immunity): These protect against pathogens in the bodily fluids (blood) and involves B-cells and antibodies Cell-mediated Immunity (T-cell Immunity): Protects against cells that have already been infected by the pathogen. Helper T-cells bind to infected cells to signal killer T-cells to come and attack the infected cells. The killer T-cell can kill the infected cell along with the pathogen by destroying the membrane of the cell. An overview of the body s immune response system to invaders Diseases in the body can be caused by inorganic toxins, genetic disorders (i.e. autoimmune diseases), or microorganisms. Microorganisms that cause sicknesses are known as pathogens. Activation of the immune response typically occurs when a pathogen enters the body. Once a germ breaches the body s natural barriers, there are five basic steps involved in immune response: 1. Alerting of the immune system 2. Triggering the first line of defense (Innate Immune System) 3. Recruiting and triggering second line of defense (Acquired Immune System)
4. Killing and disposing of pathogen 5. Generating memory Alerting of the immune system When we receive a cut, and when invaders enter the body, cells are destroyed. The dying cells trigger an automatic response called inflammation, which includes dilated blood vessels and increased blood flow. An inflammation is the body's equivalent to a burglar alarm. Once it goes off, it draws defensive cells to the damaged area in great numbers. Increased blood flow helps defensive cells reach the place where they're needed. It also accounts for the redness and swelling that occur. Triggering the first line of defense The first defenders on the scene of a germ attack are components of your innate immune system. The white blood cells that make up the innate immune system circulate throughout the body constantly, much like police on patrol, always on the lookout for biological suspects. These patrolling white blood cells belong to the phagocyte family of cells, but they have many subtypes (just as a police force consists of detectives, sergeants, captains, and patrol officers). Phagocytes consist of macrophages, dendritic cells, and granulocytes (not discussed here). In various ways, the different types of phagocytes identify, engulf, and ingest germs and other invaders. Phagocytes lead the way in many critical innate reactions. Recruiting and triggering second line of defense: Acquired Immunity Knowing that where one harmful invader lurks more may be hiding, phagocytes sound chemical alarms to bring the more specialized immune cells of the acquired immune system to the scene. First, dendritic cells display an antigen a chemical that identifies the invader so that the appropriate immune system specialist cells are able to recognize the culprit. The specialists, which consist of B and T-cells, known collectively as lymphocytes, make up the body s acquired immune system. The acquired immune system adapts and adjusts to specific threats as the need arises, whereas the innate immune system is pre-existing and less specific. Although B-cells can recognize and respond to antigens without much assistance, T-cells require a second danger signal in the form of a biological flag, known as an MHC molecule, which an antigen-presenting cell (such as a macrophage or dendritic cell) uses to clearly designate that an invader is foreign. The phagocytes also release chemical messengers known as cytokines. These danger signals rouse the T-cells, which quickly multiply and rush to the scene. The elapsed time before these defenses arrive may be days to weeks, but repeated exposure to an invader will teach T-cells to respond more quickly. Killing and disposing of pathogen Lymphocytes mount a two-pronged attack, one directed at infected cells and the other at hostile microbes circulating in the blood. The cell-targeting attack is directed by T-cells. Killer T-cells directly kill infected cells that have been marked for destruction by the phagocytes, while helper T-cells coordinate the attack and send for reinforcements as needed. Meanwhile, B-cells produce antibodies that bind to free-floating microbes circulating in the blood so that they cannot infect other cells. Phagocytes then engulf and destroy the antibody-studded invaders. Antibodies also activate complement proteins, that destroy microbes by punching holes in them. As the battle rages at the microscopic level, you may start to be aware that something is amiss. If you ve been infected with a cold virus, for example, your throat will become sore, your eyes watery, and your sinuses congested. These are the physical signs of inflammation in epithelial cells, the buildup of fluid and cells that occurs as the immune system fights a hostile invader.
Generating memory Once all invaders and infected cells have been destroyed, the immune system patrols that once multiplied so quickly decline in number. Inflammation subsides, and symptoms gradually disappear. But certain memory B and T-cells remain (and become part of the body s innate immune system), to remember how to attack the invader if it returns. What are some common pathogens that result in immune response? Scientists now know that most biological bad guys fall into four basic categories: viruses, fungi, bacteria, and parasites. Each type of predator has its own invade-and-infect strategy. Virus Viruses are not whole cells but consist of one or more molecules of DNA or RNA. In normal cells, DNA, or deoxyribonucleic acid, carries genetic information and synthesizes RNA, or ribonucleic acid, which carries information from the nucleus to the body of the cell to assemble proteins. The viruses may be shaped like rods or spheres or may be multisided. In some ways, viruses are like biological pirates: they invade cells, hijack the internal machinery there, and start reproducing, unleashing thousands of duplicate viruses (known as clones) that invade and take over other cells. Viruses also change their appearance, or mutate, often, and they can jump from one species to another (from certain animals to people, for example). Examples of viral illnesses include: colds, flu, and other common respiratory illnesses spread from one person to another; bird flu, a new and particularly deadly type of flu caused by a virus that jumped from birds to people; rabies, a deadly nervous system disease transmitted through the bite of an infected animal; West Nile Virus, a nervous system disease, transmitted through the bite of an infected mosquito; and HIV, a retrovirus that causes AIDS by destroying cells that fight disease and infection Fungi Take a walk in the woods or stroll along the aisles of a grocery store and you ll see one type of fungus a mushroom. Fungi are actually a primitive vegetable and are found in air, soil, plants, and water. Thousands of types of fungi have been identified, including yeast, mold, and mildew, and there are surely many more types, about half of which cause disease in humans. Disease-causing fungi tend to infect moist areas of the body. Athlete s foot, for example, is a common rash that results from fungi that often lurk in a shared shower (as at the gym) and then thrive and reproduce when your feet sweat. But not all fungi cause disease; some actually help fight it. Penicillin, the antibiotic fungus that transformed the treatment of infectious diseases, was discovered when a laboratory scientist noticed that a petri dish with mold growing on it was free of bacteria. Bacteria Bacteria are single-celled organisms that may be shaped like balls, rods, or spirals. Larger and more complicated than viruses, bacteria don t need to hijack your cells to wreak havoc they can do it all on their own. Fortunately, most bacteria are helpful. Some live in the stomach and intestine and help digest food. But about one in eight bacteria can make you sick. Harmful bacteria can cause problems in three ways: some invade and attack a specific part of the body; others produce chemical poisons that cause illness; still others
multiply so much that they obstruct blood vessels or prevent the heart from functioning normally. Some examples of bacterial infections and the way in which they occur: food poisoning, caused by eating contaminated food; Streptococcus, is usually transmitted by close contact with the saliva or nasal secretions from an infected individual, typically in the form of airborne respiratory droplets, anthrax poisoning, caused by inhaling or swallowing contaminated powder; and Lyme disease, which is transmitted by infected ticks and causes muscle and joint pain. Parasites Parasites comprise single-celled protozoa and multicellular animals, such as nematodes and helminths (worms), that require a moist environment to survive and usually cause disease in humans. A person can become infected after drinking contaminated water or eating infected food, or eating with unwashed hands. One example of a parasitic infection is malaria, caused by a parasite called Plasmodium falciparum. Found in the saliva of infected mosquitos, malaria causes recurring bouts of fever and chills after someone is bitten. Although the immune system responds, parasites are capable of mounting follow-up attacks: they can go into hiding in the cells for a while and then, start replicating again when the immune defense has diminished. Taking out the trash: How are pathogens are eliminated? Phagocytosis Phagocytosis (phago = eater; cyto = cell) is the process the human body uses to destroy dead or foreign cells. The phagocyte cell surrounds the particle via pseudopodia (false feet these are extensions of the cell) and engulfs and ingests the foreign particle. The particle is then broken down by enzymes in the cell. Courtesy Ask A Biologist, ASU, http://askabiologist.asu.edu/body-depot Killer T-Cells The killer T-cells are specialized in attacking cells of the body infected by viruses and bacteria. It can also attack cancer cells. The killer T-cell has receptors that are used to search each cell it encounters. If a cell is infected, it is swiftly killed. Infected cells are recognized because tiny traces of the antigen can be found on their surface. What happens when the immune system fails? Inappropriate Self Recognition: Autoimmunity The normal function of the immune system in our bodies is to protect the body by attacking and destroying foreign microorganisms, such as bacteria and viruses. As stated before, the immune system does this by producing lymphocytes and antibodies. Under normal conditions, bacteria and viruses would be recognized as
foreign invaders and thus be attacked. Under abnormal conditions of the immune system, our immune system mistakes our own cells and tissue as foreign and attacks them. For reasons not clearly understood, the immune system begins to attack and break down its own cells and tissues. This is what is known as an autoimmune disease and is the third major category of illness in the United States (behind cancer and heart disease). There are around 80 different types of autoimmune diseases which include: lupus, rheumatoid arthritis, and chronic fatigue syndrome. Ineffective Response: Immunodeficiency Immunodeficiency disorders occur when the body s immune response is reduced or absent. Immunodeficiency disorders may affect any part of the immune system. Most commonly, such a condition occurs when the T-cells and/or B-cells do not work as well as they should or when your body doesn t produce enough antibodies. People are said to be immunosuppressed when they have an immunodeficiency disorder due to medicines that affect the immune system (such as corticosteroids). Immunosuppression is also a side effect of chemotherapy. Acquired immunodeficiency may be a complication of diseases such as HIV infection and AIDS. Malnutrition - particularly from a lack of protein - and many cancers cause immunodeficiency. Exaggerated Response: Hypersensitivity Reactions Hypersensitivity is not unlike immunodeficiency. The main difference is that the immune system is ineffective because it overreacts to an antigen instead of underreacts. The immune system responds to pollen or animal dander much more aggressively than appropriate and results in an inflammatory response. This can result in allergies and anaphylaxis. Hypersensitivity often occurs in those afflicted with autoimmune disorders. Return to Table of Contents Links: http://askabiologist.asu.edu/viral-attack https://www.filamentgames.com/projects Return to Table of Contents