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IB Standards
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Phagocytes
 Leukocytes in the blood
 These cells can identify pathogens and ingest them by
endocytosis
 The pathogens are then killed and digested inside the cell by
enzymes from lysosomes.
 A pathogen is an organism or virus that causes disease
 Phagocytes can ingest pathogens in the blood
 They can also squeeze out through the walls of blood
capillaries and move through tissues to sites of infection.
They then ingest the pathogens causing the infection.
Large numbers of phagocytes at a site of infection form
pus.
 Some pathogens are able to avoid being killed by
phagocytes, so another defense is needed.
Antibodies
 Antibodies are proteins that recognize and bind
to certain antigens. Antigens are foreign
substances that stimulate the production of
antibodies.
 Antibodies usually only bind to one specific
antigen. Antigens can be any of a wide range of
substances including cell walls of pathogenic
bacteria or fungi and protein coats of pathogenic
viruses.
 Antibodies defend the body against pathogens
by binding to antigens on surface of a pathogen
and stimulating its destruction.
Barriers To Infection
 The outer layers of the skin are tough and form a
physical barrier. Sebaceous glands in the skin secrete
lactic acid and fatty acids, which make the surface of the
skin acidic. This prevents the growth of most pathogenic
bacteria.
 Mucous membranes are soft areas of the skin that are
kept moist with mucus. Mucous membranes are found in
the nose, trachea, vagina and urethra. Although they do
not form a strong physical barrier, many bacteria are
killed by lysozyme, an enzyme in the mucus. In the
trachea pathogens tend to get caught in the sticky
mucus and cilia then push the mucus and bacteria up
and out of the trachea.
Antibiotics
 Antibiotics are chemicals produced by microorganisms, to kill or
control the growth of other microorganisms. For example, Penicillin
fungus produces penicillin to kill bacteria. Most bacterial diseases in
humans can be treated successfully with antibiotics. For example,
tuberculosis has been treated with streptomycin. There are many
differences between human cells and bacterial cells and so there
are many antibiotics that block a process in bacterial cells without
causing any harm to human cells.
 Viruses carry out very few processes themselves. They rely instead
on a host cell such as a human cell to carry out the processes for
them. It is not possible to block these processes with an antibiotic
without also harming the human cells. For this reason virus diseases
cannot be treated with antibiotics.
Production of Antibodies
 1) Antibodies are made by lymphocytes, one of the two main types
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of leukocyte.
2) A lymphocyte can only make one type of antibody so a huge
number of different lymphocyte types is needed. Each lymphocyte
puts some of the antibody that it can make into its plasma
membrane with the antigen-combining site projecting outwards.
3) When a pathogen enters the body, its antigens bind to the
antibodies in the plasma membrane of one type of lymphocyte.
4) When antigens bind to the antibodies on the surface of the
lymphocyte, this lymphocyte becomes active and divides by mitosis
to produce a clone of many identical cells.
5) The clone of cells starts to produce large quantities of the same
antibody – the antibody needed to defend the body against the
pathogen.
AIDS- A Syndrome Caused By A
Virus
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AIDS shows how vital the body’s defenses against disease are. A syndrome is in a
group of symptoms that are found together. Individuals with AIDS have low numbers
of one type of lymphocyte together with weight loss and a variety of diseases caused
by viruses, bacteria, fungi and protozoa.
HIV causes AIDS. The virus infects a type of lymphocyte that plays a vital role in
antibody production. Over a period of years these lymphocytes are destroyed and
antibodies cannot then be produced. Without a functioning immune system, the body
is vulnerable to pathogens that would normally be controlled easily.
HIV does not survive for long outside the body and cannot be easily pass through the
skin. Transmissions involves transfer of body fluids from an infected from an infected
person to an uninfected one.
Through small cuts or tears in the vagina, penis, mouth or intestine during vaginal,
anal or oral sex.
In traces of blood on a hypodermic needle that is shared by intravenous drug
abusers.
Across the placenta from a mother to a baby, or through cuts during childbirth or in
milk during breast-feeding.
In transfused blood or with blood products such as Factor VIII used to treat
hemophiliacs.
Social Implications
Antibody production
 B-cells can produce antibodies, but antibody production usually
depends on other types of lymphocyte, including macrophages and
helper T-cells.
 1) Antigen presentation
 Macrophages take in antigens by endocytosis, process them and then
attach them to membrane proteins called MHC proteins. The MHC
proteins carrying the proteins carrying the antigens are then moved to
the plasma membrane by exocytosis and the antigens are displayed on
the surface of the macrophage.
 2) Activation of helper T-cells
 Helper T-cells have receptors in their plasma membrane that can bind to
antigens presented by macrophages. Each helper T-cell has receptors
with the same antigen-binding domain as an antibody. These receptors
allow a helper T-cell to recognize an antigen presented by a
macrophage and bind to the macrophage. The macrophage passes
signal to the helper T-cell changing it from an inactive to an active state.
Antibody Production continued
 3) Activation of B-cells
 Inactive B-cells have antibodies in their plasma membrane. If these antibodies
match an antigen, the antigen binds to the antibody. An activated helper T-cell
with receptors for the same antigen can then bind to the B-cell. The activated
helper T-cell sends a signal to the B-cell, causing it to change from an inactive to
an active state.
 4) Production of plasma cells
 Activated B-cells start to divide by mitosis to form a clone of cells. These cells
become active, with a much greater volume of cytoplasm. They are then known
as plasma cells. They have a very extensive network of rough endoplasmic
reticulum. This is used for synthesis of large amounts of antibody, which is then
secreted by exocytosis.
 5) Production of memory cells
 Memory cells are B-cells and T-cells that are formed at the same time as
activated helper T-cells and B-cells, when a disease challenges the immune
system. After the activated cells and the antibodies produced to fight the disease
have disappeared, the memory cells persist and allow a rapid response if the
disease is encountered again. Memory cells give long-term immunity to a
disease.
Active and passive immunity
 Resistance to infection is called immunity. Antibodies give immunity to
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disease – this is sometimes called specific immunity, because one type of
antibody gives protection against only one disease.
Active immunity is due to production of antibodies by the organism itself
after the body’s defense mechanism have been stimulated by antigens.
An example is when infection with rubella virus causes immunity to rubella
to develop and re-infection is very rare.
Passive immunity is due to the acquisition of antibodies received from
another organism, in which active immunity has been stimulated.
Examples of passive immunity:
 During pregnancy, antibodies are passed across the placenta from mother to the
fetus.
 The first milk produced after birth, called colostrum, contains antibodies that line
the gut of newborn babies, helping to prevent infection.
 Antibodies are sometimes injected as an emergency treatment for virulent
diseases, such as rabies.
Principles of Antibody Production
 The immune system has the potential to produce a vast range of different
types of antibody – perhaps 10E15 different types. It would be impossible to
make large quantities of all of these antibodies. Instead, a few B-cells that
can make a type of antibody are produced and if these cells encounter an
antigen to which their antibody binds, they multiply to form a clone of many
cells. This is called clonal selection.
 Sometimes several different types of antibodies can bind to the same
antigen, so more than one clone of cells is formed. This is called polyclonal
selection.
 Immunity to a disease is only developed if the disease challenges the
immune system. This called the principle of challenge and response.
 These two principles do not fully explain antibody diversity. Research is
ongoing into two additional processes:
 How lymphocytes splice together DNA taken from various parts of the genome,
to produce a huge variety of genes coding for antibodies
 How rapid mutation occurs in antibody genes in lymphocytes that have been
activated by antigen binding – this gives the chance of producing antibodies that
fit the antigen better.
Vaccination
 A vaccine is a modified form of a disease-causing microorganism
that stimulates the body to develop immunity to the disease, without
fully developing the disease. Vaccines contain weakened forms of
the microorganisms, killed forms or chemicals produced by the
microorganism that act as antigens. The vaccine is either injected
into the body or sometimes swallowed. The principle of vaccination
is that antigens in the vaccine cause the production of the antibodies
needed to control the disease. Sometimes two or more vaccinations
are needed to stimulate the production of enough antibodies. The
figure (right) shows a typical response to a first and second
vaccination against a disease. The first vaccination causes a little
antibody production and the production of some memory cells. The
second vaccination, sometimes called a booster shot, causes a
response from the memory cells and therefore faster and greater
production of antibodies. Memory cells should persist to give longterm immunity.
Production of Monoclonal
Antibodies
 Large quantities of a single type of antibody can
be made using an ingenious technique.
 Antigens that correspond to a desired antibody are
injected into an animal
 Tumor cells are obtained. These cells grow and divide
endlessly.
 The B-cells are fused with the tumor cells, producing
hybridoma cells that divide endlessly and produce the
desired antibody.
 The hybridoma cells are cultured and the antibodies
that they produce are extracted and purified.
Monoclonal antibodies and body
clotting continued
 Two examples:
 Treatment of anthrax
 Anthrax is a disease caused by a bacterium that produces toxins. It
is often lethal, even when antibiotic treatments are give. Anthrax
spores have sometimes been used deliberately to infect more
people and cause death. Monoclonal antibodies are being
developed which neutralize one of the toxins and therefore sustain
the patient’s life until their immune system produces antibodies
naturally.
 Tests using a monoclonal antibodies have been developed for many
diseases, including malaria. Monoclonal antibodies are produced
that bind to antigens in malarial parasites. A test plate is coated with
the antibodies. A sample is left in the plate long enough for malaria
antigens in the sample to bind to the antibodies. The sample is then
rinsed off the plate. Any antibodies with enzymes attached that
caused a color change.
 This is called an ELISA test. It can be used to measure the level of
infection and to distinguish between different strains of malaria.
Types of Bacterial Infection
 1) Extracellular bacterial infection
 Some pathogenic bacteria invade the body and remain in the
intercellular spaces, using the nutrients there.
 Example: Streptococcus
 This group of bacteria most commonly infects the upper respiratory tract.
Streptococcus cells sometimes form an outer covering, called a capsule,
which helps them to resist the antibodies in human tissues.
 2) Intracellular bacterial infection
 Some pathogenic bacteria invade the body of the host and enter its
cells, relying on the metabolism of the host cells for some processes.
 Example: Chlamydia
 Small dense Chlamydia cells are able to survive outside host cells, but not
grow or divide. When they make contact with a host cell, they are taken in by
endocytosis. Once inside they change into larger active cells, which use ATP
and other substances produced by the host, for growth and reproduction.
Eventually these active cells become smaller and denser and are released.
They may then be dispersed and enter other host cells.