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The Immune System And
Immunization
The Immunisation Advisory Centre (IMAC
The environment contains a wide variety
of potentially harmful organisms (pathogens), such as bacteria, viruses, fungi,
protozoa and multicellular parasites, which will cause disease if they enter
the body and are allowed to multiply.
The body protects itself through a
various defence mechanisms to physically prevent pathogens from entering the
body or to kill them if they do.
The immune system is an extremely
important defence mechanism that can identify an invading organism and destroy
it.
Immunisation prevents disease by
enabling the body to more rapidly respond to attack and enhancing the immune
response to a particular organism.
Each pathogen has unique distinguishing
components, known as antigens, which enable the immune system to differentiate
between ‘self’ (the body) and ‘non-self’ (the foreign material).
However, if the same antigen is seen
again, the immune system is poised to confine and destroy the organism rapidly.
This is known as adaptive immunity.
Vaccines utilise this adaptive immunity
and memory to expose the body to the antigen without causing disease, so that
when then live pathogen infects the body, the response is rapid and the
pathogen is prevented from causing disease.
Depending on the type of infectious
organism, the response required to remove it varies.
For example, viruses hide within the
body’s own cells in different tissues, such as the throat, the liver and the
nervous system, and bacteria can multiply rapidly within infected tissues.
Lines of defence
The body’s first lines of defence are
external barriers that prevent germs from entering. The largest of all is the
skin which acts as a strong, waterproof, physical barrier and very few
organisms are able to penetrate undamaged skin.
There are other physical barriers and a
variety of chemical defences. Examples of these non-specific defences are given
below:
· Skin - a strong physical barrier, like a waterproof wall
· Mucus – a sticky trap secreted by all the surfaces inside
the body that are directly linked to the outside, also contains antibodies and
enzymes
· Cilia – microscopic hairs in the airways that move to pass
debris and mucus up away from the lungs
· Lysozyme – a chemical (enzyme) present in tears and mucus that
damages bacteria
· Phagocytes – various cells that scavenge and
engulf debris and invading organisms, which form part of the surveillance
system to alert the immune system of attack
· Commensal bacteria - bacteria on skin and gut that compete
with potentially harmful bacteria for space and nutrients
· Acid - in stomach and urine, make it hard for any germs to
survive
· Fever – elevated body temperature making conditions
unfavourable for pathogens to survive
The immune response
An immune response is triggered when
the immune system is alerted that something foreign has entered the body.
Triggers include the release of
chemicals by damaged cells and inflammation, and changes in blood supply to an
area of damage which attract white blood cells.
As blood and tissue fluids circulate
around the body, various components of the immune system are continually
surveying for potential sources of attack or abnormal cells.
Antigens and
antibodies
Antigens are usually either proteins or
polysaccharides (long chains of sugar molecules that make up the cell wall of
certain bacteria).
An antigen is a molecule that
stimulates an immune response and to which antibodies bind – in fact, the name
is derived from “antibody generators.”
Any given organism contains several
different antigens.
Viruses can contain as few as three antigens
to more than 100 as for herpes and pox viruses; whereas protozoa, fungi and
bacteria are larger, more complex organisms and contain hundreds to thousands
of antigens.
An immune response initially involves
the production of antibodies that can bind to a particular antigen and the
activation of antigen-specific white blood cells.
Antibodies (immunoglobulins; Ig) are
protein molecules that bind specifically to a particular part of an antigen, so
called antigenic site or epitope.
They are found in the blood and tissue
fluids, including mucus secretions, saliva and breast milk.
There are five classes of antibody –
IgG, IgA, IgM, IgD and IgE, which have a range of functions.
They can act as ‘flags’ to direct the
immune system to foreign material for destruction and form part of the innate /
humoral immune response.
Normally, low levels of antibodies
circulate in the body tissue fluids.
However, when an immune response is
activated greater quantities are produced to specifically target the foreign
material.
Vaccination increases the levels of
circulating antibodies against a certain antigen.
Antibodies are produced by a type of
white blood cell (lymphocyte) called B cells. Each B cell can only produce
antibodies against one specific epitope.
When activated, a B cell will multiply
to produce more clones able secrete that particular antibody.
The class of antibody produced is
determined by other cells in the immune system, this is known as cell-mediated
immunity.
Primary response
Upon exposure to a pathogen, the body
will attempt to isolate and destroy it.
Chemicals released by inflammation
increase blood flow and attract white blood cells to the area of infection.
Specialist cells, known as phagocytes, engulf the target and dismantle it.
These phagocytes then travel to the
nearest lymph nodes where they ‘present’ the antigens to other cells of the
immune system to induce a larger, more specific response. This response leads
to the production of antigen-specific antibodies.
Circulating antibodies then find the organism
and bind to its surface antigens. In this way it is labelled as the target.
This specific response is also called
the adaptive or cell-mediated immune response, since the immune system adapts
to suit the type of invader.
When the body is first exposed to an
antigen, several days pass before this adaptive response becomes active.
Upon first exposure to a pathogen,
immune activity increases, then levels off and falls.
Since the first, or primary, immune
response is slow it cannot prevent disease, although it may help in recovery.
Once antigen-specific T and B cells
(lymphocytes) are activated, their numbers expand and following an infection
some memory cells remain resulting in memory for the specific
antigens.
This memory can take a few months to
fully develop.
Secondary response
During subsequent exposures to the same
pathogen, the immune system is able to respond rapidly and activity reaches
higher levels.
The secondary immune responses can
usually prevent disease, because the pathogen is detected, attacked and
destroyed before symptoms appear.
In general, adults respond more rapidly
to infection than children.
They are able to prevent disease or
reduce the severity of the disease by mounting a rapid and strong immune
response to antigens they have previously experienced.
In contrast, children have not
experienced as many antigens and are more likely to get sick.
Memory of the infection is reinforced
and long-lived antibodies remain in circulation.
Some infections, such as chickenpox,
induce a life-long memory of infection.
Other infections, such as influenza,
vary from season to season to such an extent that even an adult is unable to
adapt.
Vaccination
Vaccination utilises this secondary
response by exposing the body to the antigens of a particular pathogen and
activates the immune system without causing disease.
The initial response to a vaccine is
similar to that of the primary response upon first exposure to a pathogen, slow
and limited.
Subsequent doses of the vaccine act to
boost this response resulting in the production of long-lived antibodies and
memory cells, as it would naturally following subsequent infections.
The aim of vaccines is to prime the
body, so that when an individual is exposed to the disease-causing organism,
their immune system is able to respond rapidly and at a high activity level,
thereby destroying the pathogen before it causes disease and reduces the risk
of spread to other people.
Vaccines vary in how they stimulate the
immune system. Some provide a broader response than others.
Vaccines influence the immune response
through the nature of the antigens they contain, including number and
characteristics of the antigens, or through the route of administration, such
as orally, intramuscular or subcutaneous injection.
For most vaccines, more than one dose
may be required to provide sustained, long-lasting protection – to be fully
immunised.
Types of immunisation
Active immunisation – body
generates its own response to protect against infection through specialised
cells and antibodies, as stimulated by vaccines. Full protection takes time to
develop but is long lasting.
Passive immunisation – ready-made
antibodies are passed directly to the person being immunised. This allows for
immediate protection, but passive immunisation may only last a few weeks or
months.
Antibodies are passed from mothers to infants
across the placenta and in breast milk, to protect the infants for a short time
after birth.
Antibodies (immunoglobulins) are also
purified from blood or in laboratories; these can be directly injected to
provide rapid but short-lived protection or treatment for certain diseases,
such as rabies, diphtheria and tetanus.
The
Immunisation Advisory Centre (IMAC) is a nationwide organisation based at The
University of Auckland.
We
provide New Zealanders with independent, factual information about
vaccine-preventable diseases and the benefits and risks of immunisation. We
also provide training for health professionals, national immunisation
coordination and policy advice and research.
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