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1.1 The Immune Defence
The Immune Defence
The human immune system is an association of organs, cells and messenger substances distributed
throughout the body which cooperate with one another in fascinating ways. Only by working together
can they fulfil their major duty: protecting the individual from innumerable pathogens which are always
present everywhere in his surroundings, and eliminating deviate or unusable cells within the body.
Without the immune system, the complicated organism we call a human being would not be able to
survive.
Coli bacteria
Itch mite
Mumps virus
The Cellular Defence
The total weight of the immune system comes to about 2 kilograms. Just under 1.5 kilograms of that is
made up of white blood cells. These comrades in the ongoing battle for human health are also called
leukocytes. The name "blood cells" is somewhat misleading, for they are found everywhere in the
intercellular spaces of the body tissues and in the lymph vessels, not only in the blood.
Phagocyte
T cell
Leukocytes
The Humoral Defence = Non-cellular Defence
The leukocytes are not left to their own devices to fight intruders. They are
supported by soluble components of the blood such as the antibodies.
Antibodies have a sophisticated mechanism for recognizing, hindering and
marking intruders so that the leukocyte troops can find them.
Antibody (model)
Messenger Substances and Toxins
A complex network of chemical processes has been spun around the leukocytes and the antibodies.
In particular, the messenger substances and toxins aid the troops in their work. Thanks to the help of
the messenger substances, the defence cells can communicate with one another. Some of the
defence cells employ toxins to kill pathogens or infected cells.
Reactions to the Immune Defence
The immune defence is constantly in action. Normally we do not notice all of this hustle and bustle unless the immune system has to fight serious infections. Then we are smitten with the symptoms of
the raging battle, e.g., fever, weakness, nausea, swelling, reddening, nasal congestion, coughing. The
resources of the entire body are then deployed to support the defensive battle. Rest, warmth, vitamin-
rich nourishment and possibly medications can help the immune system to render the pathogens
harmless.
The Immunological Memory
Pathogens which have already been beaten once have virtually no chance
to attack successfully a second time. The immune system has reserves of
memory cells. If a person is re-infected by the same pathogen, the defence
normally responds so quickly that the attackers are neutralized immediately
and the body never displays any symptoms of illness.
B-memory cell
The immunological memory can also be trained to react to a particular pathogen by employing a
protective vaccination before exposure to an infection without making the person seriously ill.
1.2 Protective Barriers
Macrophoto of the skin
The Skin
People owe a special debt of thanks to their outer "protective wall," the
skin, for keeping the frequency of infection as low as it is. The skin is
colonized by microorganisms which are the body's allies. They like the
relatively "acidic" environment of the skin (pH value 4-6) and crowd out
other, dangerous germs. As long as the skin is adequately covered with
body-friendly microorganisms, harmful germs are unable to gain a foothold.
This is one reason why it is important that the "acid protective coating" and
the oily film on the skin not be destroyed by aggressive cleaning agents.
Our important allies would be "washed away."
The skin is vulnerable in other ways. An injury, a cut, a puncture or a scrape exposes the sensitive
body tissue lying underneath. Then pathogens can enter the bloodstream directly.
Mucous Membranes
Body orifices such as the mouth, nose, eye, ear, intestine and genitals are lined with mucous
membranes which are kept constantly moist with secretions. Pathogens literally become coated in
slime and are excreted along with the substance.
Many germs are taken in with food. Secretions in the mouth and stomach help to fight off pathogens.
Saliva, for example, contains enzymes which attack bacteria, and stomach acid dissolves most of the
undesired intruders.
Germs friendly to the body settle in the mucous membranes of the
intestine. This intestinal flora keeps many harmful bacteria and fungi at bay
with its secretions.
The protection provided by the mucous membranes is limited, especially
when the number of pathogens is so great that they overwhelm the
defence. Moreover, a number of pathogens are also capable of attacking or
even penetrating the mucous membranes.
Macrophoto of the intestinal
mucous membrane
If the cold virus, for example, succeeds in surmounting the slime barrier, it attaches itself to the
mucous membrane cells, penetrates them and then multiplies inside. A host of cold viruses is quickly
formed which can be eliminated only if the immune system acts against them specifically.
1.3 Defensive Action by the Immune System
The immune defence has three tasks: localization, identification and destruction of intruders. If a
pathogen has overcome protective barriers such as the skin or the mucous membranes, it runs into
specialized Defensive Actors.
The Patrols of the Immune System: Phagocytes =
Macrophages
The phagocytes belong to the vanguard of the immune defence.
They devour more or less everything which appears foreign to
them. Since they attack all possible pathogens indiscriminately,
they are regarded as representatives of the non-specific defence.
The phagocytes are responsible for keeping bacteria, viruses or
fungi under control especially during the early phase of an
infection. If the non-specific defence succeeds in eliminating all
of the pathogens, the infection has been defeated. No
immunological memory is formed.
Animated
film
Phagocyte (model)
If, however, the non-specific defence cannot deal with the invasion of pathogens by itself,
the phagocytes send out a call for additional troops. They break the pathogens down into
pieces and present parts of them on their surface as a kind of "wanted" poster. This
wanted poster, such as a fragment of the outer shell of a bacteria, is also called an
antigen.
The Alarm System of the Phagocytes
Phagocytes employ a special technique to display antigens on their surface. They use transport
molecules of the types MHC-I and MHC-II.
The abbreviation MHC stands for major histocompatibility complex. All of the body's cells bear MHC-I
molecules on their surface. In a manner of speaking, the MHC molecules serve as identification
papers. They vary from one individual to the next and help the immune system to distinguish between
own and foreign. MHC-II molecules appear only on antigen displaying cells.
MHC molecules have a kind of depression in which the antigen is presented as if on a display platter.
This is the only way to ensure that other cells of the immune system become aware of the pathogen
and initiate appropriate defence actions. Phagocytes can also call up support by sending out
messenger substances.
The Scouts and Commanders of the Immune System: T Helper Cells
The immune system has at its disposal a gigantic army of scouts, the T helper cells. They can ferret
out the antigens, the typical properties of pathogens. And as "commanders" they coordinate important
sections of the defence to fend off an infection. T helper cells are a part of the specific immune
defence.
Antigens can appear in any imaginable form. There is a matching T helper cell for every possible
antigen, just to make sure that not a single antigen is overlooked. So there is strict division of labour:
every T helper cell recognizes only one specific antigen.
The Recognition Mechanism of the T Helper Cells
Every T helper cell bears a large number of receptors, all of the same type, all around its body. Using
these receptors, the T helper cell probes the surface of other cells and determines if the antigen which
its receptor matches is located there.
The receptor of the T helper cell basically consists of two sub-receptors, the
antigen receptor and the so-called CD4 receptor.


The antigen receptor matches a specific antigen.
The CD4 receptor fits the MHC-II molecule.
T helper cell (model)
A T helper cell recognizes an antigen only if both sub-receptors can simultaneously dock onto a
phagocyte or another cell presenting an antigen. If only the antigen receptor matches the surface
antigen of a cell, the T helper cell is deactivated. So T helper cells can react only to antigens which are
displayed to them by the body's own antigen-presenting cells. The MHC-II molecules are, so to speak,
a kind of pass.
The Activation of a T Helper Cell
If a T helper cell is to initiate defensive action against a pathogen, it must first be activated by a
phagocyte. The phagocyte makes use of the co-stimulator, a molecular "ignition key," to start the T
cell.
The phagocyte releases a flood of messenger substances with the objective of raising a powerful
immune defence. The T helper cells then divide several times, producing an armada of T helper cells
with the same properties. They swarm out to mobilize reinforcements in the fight against the pathogen.
The Weapons Factory of the Immune System: B-Cells
The T helper cells look specifically for B cells which, with the help of the
MHC-II molecule, present the targeted antigen and can manufacture the
corresponding antibodies. If a T helper cell can dock on with its
receptors, it releases messenger substances. The B cells multiply, and
"weapons factories" appear which produce large quantities of
antibodies.
Animated
film
B cell (model)
The immune system has at its disposal a huge number of B cells differing from one
another so that the corresponding antibody can be prepared for every single antigen.
There is strict division of labour here as well: every B cell can produce only a single type
of antibody. B cells are a part of the specific defence.
The Guided Weapons of the Immune System: Antibodies
Antibodies are designed in the shape of a "Y". The two arms bear
identical receptors which match a specific antigen exactly.
Antibodies are a part of the specific, humoral defence.
Antibodies (model)
The antibodies search for pathogens which bear the corresponding antigen on their surface and attach
themselves. This has two effects.


The pathogens enveloped by the antibodies (= antigen-antibody complexes) are immobilized
and can no longer threaten the body's own cells.
The pathogens are marked as hostile by the antibodies and can be destroyed by the
phagocytes.
Although antibodies differ from one another in their receptors, their "stems" are all the same (Fc
region). Phagocytes recognize the "stems," latch on to them, and devour the pathogen together with
the antibodies. A kind of rubbish pick-up, the phagocytes gradually eliminate the enemy pathogens.
The Immunological Memory
When the immune system has successfully vanquished an infection, the majority of the
combatants (T helper cells, B cells) have fallen as casualties. But some of them remain in
reserve as memory cells. If infection by a pathogen which has already been defeated once
recurs, the T- and B-memory cells are quickly reactivated, and production of appropriate
antibodies can be running at top speed again in only a brief time. So the immune defence
is prepared for the infection: the body is immune to the pathogen.
Animated
film
1.4 Active and Passive Immunization
Although the immune system is basically equipped to deal with
infections of any kind, the struggle against the pathogens does not
always run smoothly. If a person's defences have been weakened, an
infection can result in serious health impairment, or even be fatal.
While the non-specific immune defence battles pathogens immediately,
it takes 7-10 days until a specific immune defence can be set up. During
this period of time, bacteria, viruses or parasites can achieve such
strength that the specific immune defence is simply too late. This can
be prevented in many cases by a preventive vaccination.
Prof. Reinhard Kurth,
Paul-Ehrlich-Institute:
Statement on vaccines
Active Immunization
Active immunization for a particular pathogen is based on a brilliant
deceptive manoeuvre. Instead of using a dangerous pathogen, the
vaccine contains a weakened strain which the immune system cannot
distinguish from the original. The advantage: the administration of a
small quantity of weakened pathogens is normally harmless for the
person being vaccinated, but still results in the desired formation of
memory cells. They are now standing by on call and can fight the
dangerous original pathogen at any time without any noticeable delay.
B cell produces antibodies
Vaccines
Modern vaccines are extraordinarily safe; undesirable side effects are
extremely rare. But a certain risk of damage from the vaccination cannot be
excluded completely. And overly sensitive people may have an allergic
reaction.
Physicians basically use two types of vaccines for the immunization: living
vaccines and deactivated vaccines.

Living vaccines contain living bacteria or viruses which are able
to reproduce, but have been greatly weakened in their ability to
produce a disease. Living vaccines are considered to be especially
effective. However, there is an extremely slight risk that the
vaccination will cause illness. One possibility is that the weakened
pathogen may, during reproduction, convert back to the strain
which causes the illness; another is that the immune system of the
vaccinated person has previously been damaged.·

Deactivated vaccines contain bacteria or viruses which have
been killed, or elements of these germs. So there is no
reproduction of the pathogen in the body. There is practically no
risk of becoming ill from deactivated vaccines.
Reproduction of yellow fever
virus in chicken eggs
Vaccination
Deactivated vaccines consist only of parts of the shell or of antigens of the pathogen. Whatever may
be hidden behind this "wanted" poster, the immune defence now has its "enemy" and reacts exactly as
if it were having to deal with the original strain of the pathogen which causes the disease. This is why it
basically makes no difference if a vaccine contains weakened or dead pathogens.
Vaccination Reactions
When the body's immune system reacts to a vaccine, symptoms may appear for a short time: e.g.


Reddening and swelling at the point of the injection
Fever, headaches and joint pain, weakness, sick feeling, nausea, swelling of the lymph nodes
Such reactions to a vaccination are completely normal. It is necessary to consult a physician only if the
symptoms continue for a longer period of time.
Booster Vaccinations
A protective vaccination does not last indefinitely. Normally the memory cells are maintained for a
maximum of ten years. When booster vaccinations are given in time, the immunity against pathogens
remains effective for a long time.
Antibodies (model)
Passive Immunization
Passive immunization involves the injection of antibodies which
match the antigens of a pathogen. These antibodies are also
known as immunoglobulins. They are normally derived from the
blood of people or animals which have already successfully beaten
off the pathogen. The foreign antibodies keep bacteria or viruses
under control until the specific defence of the infected person can
produce its own antibodies.
Passive immunization can lessen the virulence of the disease, often even save the sufferer's life.
However, the effects are not permanent. After three months at the latest, the injected antibodies have
been broken down. Unlike the active immunization, the passive immunization does not result in the
formation of memory cells. So there is no long-term protection.
1.5 The Lymphatic System
The lymphatic system of organs and vessels is of decisive importance for
the immune defence. Important defence cells, primarily the lymphocytes,
operate in the widely branching transport network which goes through the
entire body. The white blood cells help to render undesired intruders
such as viruses, bacteria and fungi harmless.
Primary Lymphatic Organs
The lymphocytes develop in the primary lymphatic organs, the bone
marrow and the thymus gland. All of the lymphocytes develop in the bone
marrow, B cells mature in the bone marrow, T cells in the thymus gland.
Secondary Lymphatic Organs
Tissue in which the defence cells become active is known as secondary
lymphatic organs. They include lymph nodes, tonsils, the spleen and
Peyer's plaques in the small intestine.
The lymphatic system
1.6 Defence Cells and Defence Substances
The defence cells (leukocytes = white blood cells) develop in the blood marrow from stem cells. The
defence cells can move through the body similarly to one-cell organisms. The table below provides an
overview of the important front-line fighters of the immune system.
Phagocytes
They track down antigens and present
them to the other cells of the immune
system so that an immune reaction is
launched as needed.
T-Helper Cells
They recognize antigens, such as those
presented by phagocytes, and can
activate B cells and cytotoxic T cells.
Cytotoxic T Cells
They destroy cells infected by viruses
and tumour cells on command.
Natural Killer Cells
They non-specifically attack virusinfected cells and tumour cells.
B-Cells
They produce antibodies on command
to fight pathogens.
Mastocytes
Defence cells to fight parasites. They
are especially involved in allergic
reactions.
The Function of the Defence Cells
The immune system has at its disposal an enormous arsenal of defence cells specialized in various
tasks which it can use in the fight against pathogens. This division of labour is highly practical as
pathogens can be found in various organs of the body and cannot be attacked in the same way at
every location.
A table gives an overview of the most important cells and their functions.
Monocytes
Precursor of the macrophages
Macrophages
(large phagocytes)
Devour intruders in all tissues and in the lymph
Cells presenting antigens
e. g. macrophages, B cells and dendritic cells, present antigens so
that an immune reaction is launched
Granulocytes
Neutrophile granulocytes (small
phagocytes)
Devour bacteria, viruses and fungi in the blood
Eosinophile granulocytes
Defence cells against parasites, involved in allergic reactions
Basophile granulocytes
(called mastocytes in the
intercellular spaces)
Defence cells against parasites, involved in allergic reactions and
inflammatory reactions
Natural killer cells
Attack non-specifically virus-infected cells and tumour cells
B cells
B lymphocytes
Precursor of the plasma cells
Plasma cells
Produce antibodies
B memory cells
Long-living B cells with antigen memory
T cells
T helper cells
Recognize antigens on cells presenting antigens, activate B cells
and cytotoxic T cells
Cytotoxic T cells
Recognize and destroy body cells and tumour cells infested with
viruses
T supressor cells
Suppress the immune response, inhibit the function of B cells and
other T cells
T memory cells
Long-living T cells with antigen memory
Components of the Defence
Non-specific defence
Specific defence
Cellular Defence
Humoral (Non-cellular)
Defence
phagocytes
(makrophages),
neutrophile granulocytes,
natural killer cells
complement, cytokines,
lysozyme
cytotoxic T cells, T helper
cells (coordination), B
cells, cells producing
antibodies, dendritic cells
antibodies
Messenger Substances (Cytokins)
The defence cells of the immune system communicate with the help of the messenger substances.
Messenger
substance
Producing cells
Function
Interleukin 1
Macrophages (phagocytes), B and Brings about the maturation of B cells to
T cells, natural killer cells
plasma cells, promotes the activity of
phagocytes and natural killer cells, lures
inflammation cells and creates fever
Interleukin 2
Activated T cells
Triggers the multiplication and
differentiation of B and T cells, activates
killer cells, etc.
Interleukin 4
Activated T cells
Promotes the formation of antibodies
Beta-Interferon
Leukocytes, fibroblasts
Contributes to suppression of the immune
response
Gamma-Interferon
T cells,
natural killer cells.
Increases the activity of phagocytes and
natural killer cells, stimulates the
multiplication of B cells and raises the
defence readiness against viruses
Tumour necrosis
factor
Macrophages (large phagocytes),
T cells
Activates B cells, T cells and phagocytes,
destroys tumour cells and causes fever
Antibodies (Immunoglobulins)
The immune system can recognize pathogens on the basis of
characteristic surface properties. These surface properties, comparable
to fingerprints, are known as antigens. The immune system forms
antibodies whose receptors match the antigens as defensive actions
against pathogens. There are several classes of antibodies which differ in
form and function (see table).
Antibody
Picture
Class Function
IgG
Immunoglobulin G (75%) is the most frequent type of
antibody. IgG is formed about three weeks after the initial
infection. If there is a following infection caused by the same
pathogen, IgG antibodies are produced in large quantities
within a brief span of time.
The mother's IgG antibodies pass through the placenta and
enter the blood circulation of the unborn child. This gives the
newborn children their initial protection from pathogens.
IgA
Immunoglobulin A (15%) fights antigens on the surfaces of
mucous membranes (e.g., in the respiratory tract and in the
intestine).
During breast feeding, IgA antibodies are passed on with the
mother's milk and strengthen the child's defences.
IgM
Immunoglobulin M is the first one produced in the case of an
infection. This is why it is also known as the early antibody.
IgD
The IgD antibody is found on the surface of B cells and takes
part in their activation. There is only a small quantity of IgD
antibodies in the blood.
IgE
Immunoglobulin E serves to fight off worm infections and
plays an important role in allergic reactions. IgE antibodies
are normally found only in traces.
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