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Igs and the Immune System
– a Short Primer
By the Health and Education Sub-Committee
I
n most discussions of
animal immunity and
vaccinations you will come
across references to ‘Ig’.
This is an abbreviation
for ‘immunoglobulin’.
An immunoglobulin, or
Ig, is a protein molecule
that is produced by an
animal’s immune system
to help recognize invading
organisms (bacterial,
viral, fungal, and parasitic
worms) and mark them for
elimination by other cells
of the immune system. Igs
are more commonly known
as ‘antibodies’. Measuring
the Ig level, or quantity of
antibodies, in the blood
therefore provides a good
indication of the health and
capabilities of the immune
system.
A B cell is triggered when it encounters its
matching antigen.
The B-cell engulfs the antigen and
digests it,
then it displays antigen fragments
bound to its unique MHC
molecules..
This combination of antigen
and MHC attracts the help
of a mature,
matching T cell.
Cytokines secreted
by the T cell help the
B cell to multiply and
mature into antibody
producing plasma
cells.
newborn cria within the
first 12 hours after birth.
Absorption of antibodies in
the gut is only possible in
the first 24 hours after birth.
Thereafter components in
the colostrum will still have
a local protective action in
the gut of the cria, but the
antibodies will no longer be
absorbed into the blood of
the cria. If the baby does not
get any colostrum, or only a
small amount of colostrum, it
is very vulnerable to infection
and is much more likely to
die. A plasma transfusion
can save such an at-risk
cria by providing it with
the antibodies of an adult
animal.
Vaccination presents the
immune system with a
sample of what some of
When a baby alpaca or llama
the common and deadly
Released into the blood,
is born, it has no antibodies
infectious organisms look
antibodies lock onto matching
at all, and is therefore very
like. This induces the
antigens. the antigen-antibody complexes are
vulnerable to infection. The
immune system to create
then cleared by the complacent cascade or by
mother’s initial colostrum
‘memory cells’, which are
the liver and spleen.
milk contains a large number
antibody (Ig) producing cells
of antibodies (that the mother
that circulate in the blood
has produced) to be passed
and watch for more invaders.
on to the young. These antibodies provide protection in the
The memory cells can last for months or years. By giving
weeks and months after birth while the cria’s own immune
periodic booster vaccinations, we refresh the population of
system is developing. The stronger the mother’s immune
memory cells, keeping the immune system primed to fight off
system, the better the protection that the baby receives via
invaders.
the colostrum. This is why it is recommended to give the
mother a vaccine booster shot a few weeks before birth, as
The primary components of the immune system are the
it enhances her antibody levels against common paddock
leukocytes, or ‘white blood cells’, that are manufactured in the
diseases. This improved protection is passed on to the cria
bone marrow and released into the blood stream. Leukocytes
via the colostrum.
come in many varieties, but they are all designed to work
together to identify foreign invaders and destroy them. The
It is essential that the colostrum reaches the gut of the
combined mass of cells of the immune system is about the
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same as the liver, making the leukocytes, as a group, one of
the largest ‘organs’ in the body.
A subtype of the leukocyte is the lymphocyte. Broadly
speaking, there are two different classes of lymphocytes, the
T-lymphocytes (T-cells) and B-lymphocytes (B-cells). The
T-cells are involved in what are known as ‘cell mediated
responses’. They respond to cells which have foreign bodies
displayed on their surface and can eliminate virus-infected and
cancerous cells. T-cells can also respond to chemical signals
to activate and search for invading bacteria. The B-cells are
involved in the production, presentation, and retention of
antibodies.
Immunoglobulins are the primary tools of the B-cells.
Specialized B-cells wander the body looking for cells or
particles that are foreign. Finding an invader is only the first
step. The rest of the immune system must be notified and
mobilized. Almost any large molecule (protein, carbohydrate,
fat) can induce an immune response. These molecules are
known as antigens, which stands for ‘antibody generators’, and
a single bacterium or virus may be covered by thousands of
them. The antigens are chopped up into small pieces, and those
pieces are then presented to the antibody-producing cells.
Every day alpacas produce millions of different antibodyproducing cells in the bone marrow. Each of these cells
produces a different, unique antibody. The end-tips of the
antibodies have what is known as a ‘variable region’ that
is different for every antibody and forms a unique threedimensional shape. If that antibody finds an antigen that has the
mirror inverse of the shape at the variable-tip, then the antibody
and antigen will bind to on another like a key fitting into a
lock. When that happens the B-cell that makes that particular
antibody knows it has a ‘winner’ and starts to mass-produce
that antibody. An invader coated in antibodies can be easily
identified and eliminated. The antibodies coating the surface
of invading bacteria or virus often have an additional effect
of preventing them from attacking the body or reproducing,
speeding their elimination. A single infectious organism can
trigger the production of dozens or even hundreds of different
antibodies against it, as there will be lots of different ‘fits’ since each organism has many different antigens on its surface.
These first antibodies vary in their efficiency for targeting the
foreign invader, but as time goes on the response will ‘mature’
as the immune system adapts and produces antibodies better
able to target the invading antigens.
Vaccines are mixtures of antigens that can activate the immune
system without causing an infection. One common tool for
making a vaccine is to use heat-killed bacteria or viruses. The
immune system is thus presented with the shell of the invader,
and can start producing antibodies against those antigens.
Because we are exposed to millions of different antigens,
the immune system has a method to distinguish occasional
threats from persistent threats. If the immune system wasted
all its efforts mass-producing antibodies against infections it
would never experience again, it might not be ready for the
more dangerous diseases. For this reason there are memory
cells. When an infection runs for a long time, or if a creature
gets infected by the same disease again and again, some of
the best antibody-producing cells will convert into memory
cells. These are antibody-producing cells that are effectively
in storage. They drift through the bloodstream, often for
years, waiting and watching for the invader for which they
optimized to reappear. If that invader comes back, the memory
cell immediately kicks into gear, makes thousands of copies
of itself, starts cranking out antibodies, and mobilizes the rest
of the immune system against the attacker. This speeds the
response time of the immune system from weeks to days, or
even hours.
Vaccinations are typically given in two doses four to six
weeks apart because this is the best method to activate the
immune system (first dose), and then cause a maturation of
the response and the production of memory cells (second
dose). Booster shots cause the memory cells to activate and
reproduce, increasing their numbers in the bloodstream. This
is necessary because the memory cells do eventually die,
so without periodic boosters the immunity would gradually
fade.
For most human vaccines we know how long the memory
cells against various antigens last, and thus doctors prescribe
different periods for booster shots (3 years, 10 years, etc.). In
camelids, the lifespan of the memory cells is not known, but
as the common vaccines are known to only provide about 12
months of protection in cattle and sheep it is recommended
that booster shots be given at least once and preferably twice
per year to ensure protection.
If you ever give a new type of vaccine (such as switching from
5 in 1 to 10 in 1) you need to give two doses, about six weeks
apart, to trigger the production of the memory cells.
New Zealand Alpaca acknowledges the input of CVG (the
Camelid Veterinary Group) in the development of this
article.
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