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The Immune System
Diseases
A disease is any change, other than injury, that disrupts the normal functions of the
body.
Some diseases are inherited while others are caused by disease causing
microorganisms called pathogens.
Infectious diseases are caused by pathogenic microorganisms, such as bacteria,
viruses, parasites or fungi; the diseases can be spread, directly or indirectly, from one
person to another. Zoonotic diseases are infectious diseases of animals that can cause
disease when to transmitted humans.
Germ Theory of DiseaseUnlike the conventional explanation of diseases, Louis Pasteur and Robert Koch
concluded that infectious diseases were caused by microorganisms of different types
known as germs. This is known as the Germ Theory of Diseases.
Koch gave a set of rules to identify the microorganism that cause a specific disease1. The pathogen should always be found in the body of a sick organism and
should not be found in a healthy one.
2. The pathogen must be isolated and grown in laboratory in pure culture.
3. When the cultured pathogens are placed in a new host, they should cause the
same disease that infected the original host.
4. The injected pathogen should be isolated from the second host. It should be
identical to the original pathogen.
Agents of DiseaseThe common agents of disease are –
1. Viruses- They are tiny particles that invade and replicate living cells. They
insert their genetic material into the host cells and take over many of their
functions. They can infect nearly every type of living organisms. Examples of
viral diseases are- common cold, influenza, small pox, etc.
2. Bacteria – Some bacteria cause illness. They can cause illness by either
breaking down cells for their food or releasing toxins that harm the body.
Example of bacterial infection- streptococcus, diphtheria, anthrax, botulism.
3. Protists- protests are microscopic plants or animals that live as parasites in a
host cell. Example- Malaria causes by protests plasmodium, African sleeping
sickness caused by trypanosoma , dysentery caused by Entamoeba.
4. Worms – stay as parasites in a living cell. Example- hookworm, roundworms,
tapeworms, etc.
5. Fungi- Most fungi are harmless but some are potential dangers. They can
penetrate the outer layers of skin. Example- athelete’s foot.
How Diseases are SpreadSome infectious diseases are spread from one person to another through coughing,
sneezing, or physical contact. Other infectious diseases are spread through
contaminated water or food. Still others are spread by infected animals.
Fighting Infectious Diseases Antibiotics-Many antibiotics are produced naturally by living organisms. First
antibiotic- Penicillin was made from a mold called penicillium by Alexander
Fleming. They interfere with the growth of bacteria. They are ineffective
against viruses.
 Anti-viral drugs- These drugs generally inhibit the ability of viruses to invade
cells and to multiply once inside cells.
 Over-the –counter drugs- These drugs treat only the symptoms of the disease.
They help you feel better but do not actually treat the cause of the infection.
The Immune SystemThe immune system is a system that fights infection through the production of
cells that inactivate foreign substances or cells. This process is called immunity.
The immune system includes two general categories of defense mechanisms
against infection: non specific defenses and specific defenses.
Non Specific DefensesNon specific defenses do not discriminate between one threat and another. These
defenses include physical and chemical barriers.
1. First Line of Defense The function of the first line of defense is to keep pathogens out of the
body. This role is carried out by skin, mucus, sweat and tears.
 Very few pathogens can penetrate the layers of dead cells at skin’s surface.
 Many secretions of the body, including mucus, saliva, and tears, contain
lysosome, an enzyme that breaks down the cell walls of many bacteria.
 Sweat glands produce an acidic environment that kills many bacteria.
 Mucus in nose and throat helps to trap pathogens.
 The cilia in nose and throat help to push the pathogens away from your
lungs.
 Stomach acid and digestive enzymes destroy many pathogens that reach
stomach.
2. Second Line of DefenseInflammation - When pathogens enter the body, they multiply quickly, releasing
toxins into the tissues. This activates the inflammatory response. It is a non-specific
defense reaction to tissue damage caused by injury or infection.
Upon detection of the pathogen, the immune system produces millions of W.B.C’s ,
which fight the infection. Blood vessels near the wound expand and W.B.C’s move
from the vessel to enter the infected cells. Many of these W.B.C’s are phagocytes
which engulf and destroy bacteria. The infected tissue may become swollen and
painful.
Fever- The second type of second line of defense is Fever. Fever is the increase in
core body temperature due to the release of certain chemicals. Many pathogens can
survive only within a narrow temperature range. An elevated temperature slows down
or stops the growth of such pathogens. The higher body temperature also increases the
heart rate so that white blood cells get to the site of infection faster.
3. Interferon- Interferon are a specific group of proteins that are produced by viral
infected cells to help other cells resist viral infection. They inhibit the synthesis of
viral proteins in infected cells and help block viral infections. This process slows
down the progress of infection and often gives the specific defenses of the immune
system time to respond.
Specific DefensesWhen non- specific responses like skin, sweat, saliva, mucus etc are not able to stop
the pathogens, the immune system reacts with a series of specific defenses that attack
the particular disease-causing agent. These defenses are called the immune response.
A substance that triggers this response is known as an antigen. For example: bacteria,
viruses, fungi, protests etc.
There are two types of immune responses in body:
1. Humoral Immunity
There are two types of lymphocytes in the blood- B lymphocytes ( B Cells) and T
lymphocytes( T Cells). When the specific antigens are recognized by B-Cells , the
immunity is known as humoral immunity. These B- Cells provide immunity
against antigens and pathogens in the body fluids.
How Does Humoral Immunity WorkUpon recognizing the antigens, these B-Cells grow and divide rapidly, producing
large numbers of plasma cells and memory B cells.
Plasma cells release antibodies. Antibodies are proteins called immunoglobins that
recognize and bind to the antigens. The antibodies are carried in the bloodstream
to attack the pathogen that is causing the infection. Once the infection is
controlled and treated, the plasma cells die out and stop producing antibodies.
Now, the memory B cells remain capable of producing antibodies specific to that
antigen. If the same antigen enters the body a second time, a secondary response
occurs. The memory B cells divide rapidly forming new plasma cells that will
produce the specific antibodies to destroy the pathogen.
Antibody StructureAntibody is shaped like the letter Y and has two identical antigen binding sites.
The different shapes of the binding site make it possible for the antibodies to
recognize a large variety of antigens.
2. Cell- mediated immunity
This type of immunity is provided by T- Cells. T- Cells provide a defense against
abnormal cells and pathogens inside living cells. Cell- mediated immunity is
important in destroying cells that have become cancerous or infected by viruses
and fighting the infections caused by bacteria and viruses.
During cell- mediated immunity, T-cells divide and differentiate into killer T cells,
Helper T cells, suppressor T cells, and memory T cells.
Killer T cells- They track down the bacteria, fungi, protozoan or foreign tissue
that contains the antigen.
Helper T cells- They produce memory T cells.
Memory T cells- They cause a secondary response if the same antigen enters the
body again.
Suppressor T cells- They shut down the killer T cells when their job is done.
T Cells and Organ TransplantationIn the cases, where organs need to be transplanted, T cells make the acceptance of
the organ to be transplanted difficult. The T cells recognize the organ as a foreign
body and attacks it. To overcome this problem, doctors search for a donor whose
cell markers are nearly identical to the cell markers of the recipient. Recipients
usually have to drugs through out their lives to suppress the cell- mediated
immune responses.
(Watch this animation for working of immune systemhighered.mcgraw-hill.com/.../animation__the_immune_response.html)
Acquired Immunity
1. Active Immunity – The type of immunity produced by the body’s reaction to
a vaccine is known as active immunity. Vaccination is the injection of a
weakened form of a pathogen to produce immunity.
In active immunity, the body makes its own antibodies in response to an
antigen. It appears after exposure to an antigen, as a result of the immune
response. It may develop as a result of natural exposure or vaccine.
2. Passive Immunity- When antibodies produced by other animals against a
pathogen are injected into the blood stream , the antibodies produce a passive
immunity against the pathogen. It lasts only a short time because the body
eventually destroys the foreign antibodies. It can also develop naturally or by
deliberate exposure. Example of natural passive immunity is transfer of
antibodies from mother to fetus. Example of deliberate passive immunity is
shots of rabies after bitten by an infected animal.
Immune System Disorders:
There are three different kinds of immune system disorders:
1. Allergies –
What are allergies - Allergies are the over reactions of immune system.
Antigens that cause allergies are called allergens.
What Happens when you have an allergy – When allergy causing antigens
enter the body, they attach to the mast cells (specialize immune cells) that
initiate the inflammatory response. The activated mast cells release chemicals
called histamines.
Symptoms:
 They increase the flow of blood and fluid to the surrounding area
so the area looks red and feels warm.
 They increase mucus production in the respiratory system that
brings in sneezing, watery eyes, runny nose.
Treatment : By taking antihistamines.
Asthma – is a dangerous allergic condition. It is a chronic respiratory disease
in which air passages cause wheezing, coughing and difficulty in breathing. It
can be either heredity or acquired.
Asthma attacks can be triggered by respiratory infections, exercise, emotional
stress and certain medications. It can also be triggered by cold air, pollen, dust,
tobacco smoke, pollution, molds, and pet dander.
Treatment : There is no cure for asthma. Medications can be used to relieve
symptoms.
2. Autoimmune Diseases – When the immune system makes a mistake and
attacks the body’s own cells, it produces an autoimmune disease. In an
autoimmune disease, the immune system produces “ antiself ”antibodies.
Examples:
 Type I diabetes- antibodies attack the insulin producing cells of the
pancreas.
 Rheumatoid arthiritis- antibodies attack connective tissue around the
joints.
 Myasthenia gravis- antibodies attack neuromuscular junctions.
 Multiple sclerosis- antibodies destroy the functions of the neurons in
the brain and spinal cord.
Treatment : These disease are treated with medications that alleviate
specific symptoms.
3. Immunodeficiency diseases
They are of two types-the diseases in which the immune system fails to
develop normally and AIDS.
AIDS – Acquired Immunodeficiency Syndrome
AIDS results from a viral infection that destroys helper T cells. The decline of
helper T cells leads to the break down of the immunity system.
Cause – AIDS is caused by a virus called HIV ( Human Immunodeficiency
Virus) . HIV is a retrovirus- a virus that carries its genetic information in RNA
rather than DNA. HIV evades the defenses of the immune system. It also
attacks the key cells in the immune system, destroying the body’s defenses
and leaving the body with no protection against other pathogens.
What happens in AIDS HIV targets the helper T cells.
 After that, it attaches itself to receptor molecules on the cell membrane.
This allows the virus to enter the cell.
 Once the viral core is inside the cell, it forces the host cell to make
DNA copies of the virus’s RNA.
 Some of these copies insert themselves into host cell DNA and stay
there permanently. Other copies remain in the cytoplasm. The viral
DNA may remain inactive in the host cell for varying periods of time.
 When activated, the viral DNA directs the production of viral RNA
and proteins that are assembled into new virus particles. These viruses
eventually leave the infected cell and infect new cells.
 The antibodies produced by the immune system are not effective
against these viruses.
 Eventually the number of helper T cells decreases leaving the body
more and more susceptible to other diseases.
Transmission of HIV –
There are four main ways that HIV can be transmitted:
 through any form of sexual intercourse with an infected person;
 through shared needles or syringes that are contaminated with the
blood of an infected person;
 through contact with blood or blood products of an infected person;
 from an infected mother to child, either during pregnancy, during birth,
or during breast-feeding.
Cure and TreatmentThere is no cure for AIDS. The only way to control the virus is to use expensive
multidrug and multivitamin “cocktails” that fight the virus in several ways.
Resource: Textbook- Biology- Miller and Levine