Cambridge International A Level Biology Revision Guide

Chapter 11: Immunity
Summary
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Phagocytes and lymphocytes are the cells of the
immune system. Phagocytes originate in the bone
marrow and are produced there throughout life. There
are two types of phagocyte. Neutrophils circulate in the
blood and enter infected tissues; macrophages remain
inside tissues. Neutrophils and macrophages destroy
bacteria and viruses by phagocytosis.
Antigens are ‘foreign’ (non-self) molecules that
stimulate the immune system.
Lymphocytes also originate in bone marrow. There are
two types: B-lymphocytes (B cells) and T-lymphocytes
(T cells). B cells and T cells gain glycoprotein receptors
that are specific to each cell as they mature. Each
lymphocyte divides to form a small clone of cells that
spreads throughout the body in the blood and in the
lymphoid tissue (e.g. lymph nodes and spleen). B cells
mature in bone marrow. T cells mature in the thymus
gland. During maturation, many T cells are destroyed, as
they express receptors that interact with self-antigens. If
left to circulate in the body, they would destroy cells and
tissues. The T cells that are not destroyed recognise nonself
antigens, such as those on the surfaces of pathogens.
During an immune response, those B and T cells that
have receptors specific to the antigen are activated.
When B cells are activated, they form plasma cells which
secrete antibodies. T cells do not secrete antibodies;
their surface receptors are similar to antibodies and
identify antigens. T cells develop into either helper T
cells or killer T cells (cytotoxic T cells). Helper T cells
secrete cytokines that control the immune system,
activating B cells and killer T cells, which kill infected
host cells.
During an immune response, memory cells are formed
which retain the ability to divide rapidly and develop
into active B or T cells on a second exposure to the same
antigen (immunological memory).
Antibodies are globular glycoproteins. They all have
one or more pairs of identical heavy polypeptides and
of identical light polypeptides. Each type of antibody
interacts with one antigen via the specific shape of
its variable region. Each molecule of the simplest
antibody (IgG) can bind to two antigen molecules.
Larger antibodies (IgM and IgA) have more than two
antigen-binding sites. Antibodies agglutinate bacteria,
prevent viruses infecting cells, coat bacteria and viruses
to aid phagocytosis, act with plasma proteins to burst
bacteria, and neutralise toxins.
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Blood cell counts give useful information in diagnosis
and in monitoring the success of treatments for certain
medical conditions. During infectious diseases, the
white blood cell count often increases as neutrophils are
released from stores in the bone marrow (neutrophils
travel in the blood to the site(s) of infection to destroy
the pathogens by phagocytosis).
Leukaemias are cancers of the stem cells in the
bone marrow that give rise to blood cells. 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.
Active immunity is the production of antibodies and
active T cells during a primary immune response
to an antigen acquired either naturally by infection
or artificially by vaccination. This gives long-term
immunity. Passive immunity is the introduction of
antibodies either naturally across the placenta or
in breast milk, or artificially by injection. This gives
temporary immunity.
Vaccination confers artificial active immunity by
introducing a small quantity of an antigen by injection
or by mouth. A vaccine may be a whole living organism,
a dead one, a harmless version of a toxin (toxoid) or
a preparation of antigens. It is difficult to develop
successful vaccines against diseases caused by
organisms that have many different strains, express
different antigens during their life cycle within humans
(antigenic variation), or infect parts of the body beyond
the reach of antibodies (antigenic concealment).
Smallpox was eradicated by a programme of
surveillance, contact tracing and ‘ring’ vaccination,
using a ‘live’ vaccine against the only strain of the
smallpox virus.
Measles is a common cause of death among infants in
poor communities. It is difficult to eradicate because
a wide coverage of vaccination has not been achieved
and malnourished children do not respond well to just
one dose of the vaccine. Vaccines for TB and cholera
are not very effective. Vaccines for malaria are being
trialled and may become available for widespread use.
Antibodies are not very effective against V. cholerae and
its toxin, as the bacterium and its toxin remain in the
alimentary canal. Antibodies cannot cross cell surface
membranes to reach M. tuberculosis, which infects host
cells, especially macrophages.
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