Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
232
All the white cells in the blood originate from stem
cells in the bone marrow. There are two groups of bone
marrow stem cells:
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myeloid stem cells that give rise to neutrophils,
monocytes and platelets
lymphoid stem cells that give rise to lymphocytes, both
B and T cells.
These stem cells divide rapidly to produce huge numbers
of mature, differentiated blood cells that function in
specific parts of the immune response. Leukaemias are
cancers of these stem cells. The cells divide uncontrollably
to give many cells which do not differentiate properly and
disrupt the production of normal blood cells including red
blood cells and platelets. These malignant cells fill up the
bone marrow and then flow into the blood and into the
lymphatic system.
In myeloid leukaemias, the stem cells responsible for
producing neutrophils divide uncontrollably and the
number of immature cells increases. In lymphoblastic
leukaemias, the cancerous cells are those that give rise
to lymphocytes.
The immature white blood cells are produced
very quickly and they disrupt the normal balance of
components in the blood. This means that the body
does not have enough red blood cells or platelets. This
causes anaemia and increases the risk of excessive
bleeding. Also, the numbers of mature neutrophils and
lymphocytes decrease so that people with these cancers
become more susceptible to infections; they are said to be
immunosuppressed.
There are acute and chronic forms of both types of
leukaemia. Acute leukaemias develop very quickly, have
severe effects and need to be treated immediately after they
are diagnosed; chronic leukaemias may take many years
to develop and changes in blood cell counts are usually
monitored over time so that treatment is given when it is
most likely to cure the disease. Blood tests are used to help
diagnose these diseases, monitor their progress and assess
the effectiveness of treatments.
Active and passive immunity
The type of immunity described so far occurs during the
course of an infection. This type of immunity is called
active immunity because the person makes their own
antibodies. This happens when the lymphocytes are
activated by antigens on the surface of pathogens that have
invaded the body. As this activation occurs naturally during
an infection it is called natural active immunity.
The immune response can also be activated
artificially, either by injecting antigens into the body
or – for certain diseases such as polio – taking them by
mouth. This is the basis of artificial active immunity,
more commonly known as vaccination. The immune
response is similar to that following an infection, and the
effect is the same – long-term immunity. In both natural
and artificial active immunity, antibody concentrations
in the blood follow patterns similar to those shown in
Figure 11.8 (page 228).
In both forms of active immunity, it takes time for
sufficient numbers of B and T cells to be produced to give
an effective defence. If a person becomes infected with a
potentially fatal disease such as tetanus, a more immediate
defence than that provided by active immunity is needed
for survival. Tetanus kills quickly, before the body’s
natural primary response can take place. So people who
have a wound that may be infected with the bacterium that
causes tetanus are given an injection of antitoxin. This
is a preparation of human antibodies against the tetanus
toxin. The antibodies are collected from blood donors who
have recently been vaccinated against tetanus. Antitoxin
provides immediate protection, but this is only temporary
as the antibodies are not produced by the body’s own B
cells and are therefore regarded as foreign themselves.
They are removed from the circulation by phagocytes in
the liver and spleen.
This type of immunity is called passive immunity
because the person has not produced the antibodies
themself. B and T cells have not been activated, and
plasma cells have not produced any antibodies. More
specifically, antitoxins provide artificial passive
immunity, because the antibodies have not entered the
body by a natural process: they have come from another
person who has encountered the antigen.
The immune system of a newborn infant is not as
effective as that of a child or an adult. However, infants
are not entirely unprotected against pathogens, because
antibodies from their mothers cross the placenta during
pregnancy and remain in the infant for several months
(Figure 11.14). For example, antibodies against measles
may last for four months or more in the infant’s blood.
Active immunity is immunity gained when an antigen
enters the body, an immune response occurs and
antibodies are produced by plasma cells.
Passive immunity is immunity gained without an
immune response; antibodies are injected (artificial)
or pass from mother to child across the placenta or in
breast milk (natural).