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Viral Attack! Your Clean-Up Crew to the Rescue!
A Gallery Demonstration at Arizona Science Center
Table of Contents
Background Information
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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.
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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.
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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.
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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.
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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.
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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.
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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:
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Humoral Immunity (B-cell Immunity): These protect against pathogens in the bodily fluids (blood) and
involves B-cells and antibodies
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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.
5.
Killing and disposing of pathogen
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;
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bird flu, a new and particularly deadly type of flu caused by a virus that jumped from birds to
people;
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rabies, a deadly nervous system disease transmitted through the bite of an infected animal;
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West Nile Virus, a nervous system disease, transmitted through the bite of an infected mosquito;
and
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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:
●
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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.
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