Immune System Our immune system is made up of: The innate
immune system: first line of defence (non-specific) The adaptive
immune system: more sophisticates defence system (specific to the
pathogen it targets).
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Innate Immunity (Non-specific) The innate immune system
responses are general defence systems of the body. i.e. They are
not specific This is the first line of defence and the second line
of defence
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Innate Immunity: The Skin The skin is the primary defence of
the body. The protective boundaries of some other human tissues
(eyes, nasal passages, respiratory tract, mouth, digestive tract,
urinary tract, and female genital tract) are less robust. In the
lungs and small intestine, the barrier is just a single monolayer
of epithelial cells. Pathogens must cross this protective barrier
in order to colonise a host. Other first line defence mechanisms
include mucous membranes, natural secretions and natural flora (of
the body).
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Second Line of Defence Once the bodys first line of defence has
been compromised, the second line of defence is employed. This may
include: Phagocytes and killer cells Complement proteins Interferon
Cytokines Inflammation
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Adaptive Immune System (Specific ) The adaptive immune system
responses are highly specific to a particular pathogen, and they
provide long-lasting protection against them. Adaptive immune
responses eliminate or destroy invading pathogens and any toxic
molecules they produce. As these responses are destructive, it is
important that they are directed only against foreign molecules and
not against molecules of the host itself. Many harmless foreign
molecules enter the body, and it would be pointless and potentially
dangerous to mount adaptive immune responses against them.
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Adaptive Immune System The adaptive immune system uses multiple
mechanisms to avoid damaging responses against self molecules.
Occasionally these mechanisms fail, and the system turns against
the host, causing autoimmune diseases, which can be fatal.
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Allergies Allergic conditions such as hayfever and allergic
asthma are examples of adaptive immune responses against apparently
harmless foreign molecules.
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Antigens Any substance capable of eliciting an adaptive immune
response is referred to as an antigen. The adaptive immune system
can distinguish between antigens that are very similarsuch as
between two proteins that differ in only a single amino acid.
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The Innate and Adaptive Immune Systems Work Together
Lymphocytes usually respond to foreign antigens only if the innate
immune system is first activated. the rapid innate immune responses
to an infection depend largely on pattern recognition receptors
made by cells of the innate immune system.
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Lymphocytes Lymphocytes are responsible for the specificity of
adaptive immune responses. They occur in large numbers in the blood
and lymph (the colorless fluid in the lymphatic vessels that
connect the lymph nodes in the body to each other and to the
bloodstream). They are also concentrated in lymphoid organs, such
as the thymus, lymph nodes (also called lymph glands), spleen, and
appendix.
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Lymphocytes
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Adaptive immune responses are carried out by white blood cells
called lymphocytes. There are two broad classes of such responses
antibody responses T-cell-mediated immune responses There are also
different classes of lymphocytes, called B cells and T cells which
carry out the above responses, respectively.
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Antibody Responses In antibody responses, B cells are activated
to secrete antibodies, which are proteins called immunoglobulins.
The antibodies circulate in the bloodstream and permeate the other
body fluids, where they bind specifically to the foreign antigen
that stimulated their production. Binding of antibody inactivates
viruses and microbial toxins (such as tetanus toxin or diphtheria
toxin) by blocking their ability to bind to receptors on host
cells. Antibody binding also marks invading pathogens for
destruction, mainly by making it easier for phagocytic cells of the
innate immune system to ingest them.
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Antibodies (Immunoglobulins)
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Immunoglobulins There are different classes of immunoglobulins
IgM antibodies first to be secreted in an infection. Cause
agglutination of cells containing antigens. IgG antibodies activate
complement proteins in the blood & neutralizetoxins. (Most
common antibody that passes between mother and baby.) IgA
antibodies neutralise pathogens in respiratory, digestive and
reproductive tracts. IgE antibodies help initiate inflammation
after pathogens infect. Also causes allergic reactions to
non-pathogenic agents.
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T-Cell Mediated Immune Responses In T-cell-mediated immune
responses, activated T cells react directly against a foreign
antigen that is presented to them on the surface of a host cell. T
cells can detect microbes hiding inside host cells and either kill
the infected cells or help the infected cells or other cells to
eliminate the microbes. The T cell, for example, might kill a virus
infected host cell that has viral antigens on its surface, thereby
eliminating the infected cell before the virus has had a chance to
replicate. In other cases, the T cell produces signal molecules
that either activate macrophages to destroy the microbes that they
have phagocytosed or help activate B cells to make antibodies
against the microbes.
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Summary A summary of the two types of adaptive immune responses
Antibody Responses T-Cell Mediated Responses
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Recognising Self and Non-self B Cells and T Cells need to be
able to distinguish between self and non-self. Self cells are
determined by proteins on the cell membrane. Proteins on cell
membranes are determined by genes These genes code for major
histocompatibility complex (MHC) and these proteins are called
markers. All cells have MHC on their surfaces. B and T cells
recognise and ignore cells that have the same MHC markers as
themselves.
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MHC Markers There are 2 types of MHC markers. Class 1 MHC
markers are found on the surface of all nucleated cells (all body
cells except red blood cells) Class 2 MHC markers are found on the
surface of B cells, T cells and macrophages. MHC markers also have
a role in governing the success of organ transplants (the immune
system is suppressed to prevent rejection of foreign tissue).
