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acquired immune system
Sometimes also called specific or adaptive immunity, acquired immunity is those features of the immune system that are "learned" during a
person's lifetime rather than the ones the individual is born with. This is the part of the immune system that deals with specific invaders and
learns to recognise them by exposure to them.
The other part of the immune system, the one that we are born with, is called the innate immune system and consists of many mechanisms
which are all non-specific - that is they are not programmed to recognise any particular invaders.
Acquired immunity is further divided into two parts:
humoral immunity which principally operates through a type of lymphocyte called a B-cell which originates in the bone marrow and is
matured both in the marrow and the spleen.
cell mediated immunity which principally operates through a type of lymphocyte called a T-cell which also originates in the bone marrow but
is matured in an organ called the thymus.
Both parts of the acquired immune system responds to peptide sequences called antigens. Antigens are sections of broken up proteins
from cells that have been ingested by several different types of leukocyte. These leukocytes then present these antigens to both B- and
T-cells and are known as antigen presenting cells (APC). Antigens are the way that the acquired immune system recognises invading
bacteria, viruses and other harmful organisms (pathogens). Both B- and T-cells have surface receptors that recognise specific antigens. All
the receptors on each individual lymphocyte are monoclonal which means that they are all the same and so each lymphocyte is
programmed only to recognise a specific antigen.
Humoral Immunity
When a B-cell locates an antigen it doesn't react with it but divides repeatedly to produce identical daughter lymphocytes. Most of these
B-cells then transform into plasma cells. These then shed their receptors in a soluble form known as antibodies and release them into the
blood and lymph fluid. However, some of the activated B-cells do not become plasma cells but become what are known as memory B-cells
which continue to produce small amounts of the antibody long after the infection has been overcome.
When an antibody binds to an antigen on a cell, it acts as a signal for neutrophils, eosinophils, basophils and macrophages to engulf and
kill (phagocytose) it in a process known as opsinisation. Another thing that happens when antibodies bind to cells is that serum proteins
known as the complement bind to the immobilized antibodies and destroy the attached cells. Antibodies also signal natural killer cells and
macrophages to kill cells that are internally infected with viruses and bacteria. Antibodies will also neutralise many pathogens simply by
binding to them.
Cell Mediated Immunity
T-cells are manufactured in the thymus where they are tested for their response to self-tissue. Any that respond to self-tissue are usually
rejected. However, it is clear that in T-cell mediated autoimmune diseases (e.g. multiple sclerosis) some T-cells with self-receptors somehow
get manufactured.
There are three main types of T-Cell - Helper T-cells, Killer T-cells and Memory T-cells and all three recognise specific antigens.
When a helper T-cell recognises its antigen it migrates to the secondary lymphoid tissue where it divides into multiple memory T-cells and
killer T-cells. It is the killer T-cells that actually destroys any cells it meets that express the antigen that its receptors match for.
T-cells are a type of white blood cell (leukocyte) that belongs to a division of the immune system called the acquired immune system. This
is the part of the immune system that learns to combat invading bacteria and viruses (pathogens) through exposure to them. The
leukocytes that belong to the acquired immune system are called lymphocytes and have receptors on their surface that recognise broken
down protein fragments, called antigens, by binding with them. Antigens can be derived either from invading pathogens or, in the case of
autoimmune diseases like multiple sclerosis, from cells of one's own body.
The acquired immune system is further divided into cell-mediated or cellular immunity and humoral or antibody-mediated immunity. T-cells
are the major cells that drive cellular immunity whereas an another type of lymphocyte, called a B-cell, is the principle cell involved with
antibody-mediated immunity. T-cells are so-called because they are matured in an organ called the Thymus.
The surface of a T-cell contains thousands of T-Cell Receptors (TCR) but, for any one T-cell, all the receptors are identical (monoclonal).
This means that any one T-cell is only able to recognise a small group of related antigens i.e. each T-cell is specific only to those antigens
and is not effective against any others. The receptor rarely binds with an entire antigen but with a sub-section of it called an epitope.
The job of T-cells is to detect cells in the body that are internally infected with viruses and bacteria. They do this by sampling the contents
of cells. Two types of T-cells sample different populations of cells and take different action when they detect a antigen. These are the
"killer" or cytotoxic (CD8+) T-cells and the "helper" (CD4+) T-cells. The CD8+ and CD4+ describe the types of receptors that each carries.
A third type of T-cell called a "suppressor" T-cell also uses the CD8+ receptor.
Almost all the cells in the body express a protein called the Major Histocompatibility Complex (MHC) protein. The function of MHC is to
present antigens to T-cells. MHC has a slit in it shaped like a letter box and the cell pushes antigens through this slit. T-cell receptors plug
onto the MHC molecule and try to bind with the presented antigen. MHC comes in two major varieties: MHC class I and MHC class II.
MHC class I is present on almost all cells and it is the job of killer T-cells to bind to antigens presented in this way. When a match is found,
the killer T-cell latches onto the infected cell and destroys it. How this is done is dealt with in the entry for killer T-cells.
MHC class II is present only on a population of cells known as antigen presenting cells (APC). These include macrophages, B-cells and
dendritic cells. It is the job of helper T-cells to bind to antigens presented in this way. When this happens, a helper T-cell can do several
things:
It produces special messenger molecules called cytokines. Various different cytokines send different complex signals to other cells
including: calling immune system cells to the site of the infection, telling endothelial (blood vessel "lining") cells to let these other cells
through and telling immune system cells to activate themselves.
Cooperate with complementary B-cells to get them to clone themselves and to release antibodies
Clone themselves to increase the number of this type of T-cell.
Helper T-cells are strongly implicated in the process of demyelination in multiple sclerosis. They are dealt with more fully in the entry for
helper T-cells.
The third type of T-cell, the suppressor T-cell, is involved with suppressing an immune response. It is not well understood how they do but
they probably use several mechanisms including "programmed cell death" (apoptosis) which involves sending cytokines to other immune
system cells telling them to commit suicide. Suppressor T-cells are dealt with more fully in the entry for suppressor T-cells.
T-cells are manufactured in the bone marrow but migrate to an organ called the thymus where they are matured via a process called
affinity maturation which removes those which are active against the body's own antigens (autoreactive). Selection for particular T-cells is
dealt with in the entry on the thymus.
is the part of the immune system that learns to combat invading bacteria and viruses (pathogens) through exposure to them. The
leukocytes that belong to the acquired immune system are called lymphocytes and have receptors on their surface that recognise broken
down protein fragments, called antigens, by binding with them. Antigens can be derived either from invading pathogens or, in the case of
autoimmune diseases like multiple sclerosis, from cells of one's own body.
The acquired immune system is further divided into cell-mediated or cellular immunity and humoral or antibody-mediated immunity. T-cells
are the major cells that drive cellular immunity whereas an another type of lymphocyte, called a B-cell, is the principle cell involved with
antibody-mediated immunity. T-cells are so-called because they are matured in an organ called the Thymus.
The surface of a T-cell contains thousands of T-Cell Receptors (TCR) but, for any one T-cell, all the receptors are identical (monoclonal).
This means that any one T-cell is only able to recognise a small group of related antigens i.e. each T-cell is specific only to those antigens
and is not effective against any others. The receptor rarely binds with an entire antigen but with a sub-section of it called an epitope.
The job of T-cells is to detect cells in the body that are internally infected with viruses and bacteria. They do this by sampling the contents
of cells. Two types of T-cells sample different populations of cells and take different action when they detect a antigen. These are the
"killer" or cytotoxic (CD8+) T-cells and the "helper" (CD4+) T-cells. The CD8+ and CD4+ describe the types of receptors that each carries.
A third type of T-cell called a "suppressor" T-cell also uses the CD8+ receptor.
Almost all the cells in the body express a protein called the Major Histocompatibility Complex (MHC) protein. The function of MHC is to
present antigens to T-cells. MHC has a slit in it shaped like a letter box and the cell pushes antigens through this slit. T-cell receptors plug
onto the MHC molecule and try to bind with the presented antigen. MHC comes in two major varieties: MHC class I and MHC class II.
MHC class I is present on almost all cells and it is the job of killer T-cells to bind to antigens presented in this way. When a match is found,
the killer T-cell latches onto the infected cell and destroys it. How this is done is dealt with in the entry for killer T-cells.
MHC class II is present only on a population of cells known as antigen presenting cells (APC). These include macrophages, B-cells and
dendritic cells. It is the job of helper T-cells to bind to antigens presented in this way. When this happens, a helper T-cell can do several
things:
It produces special messenger molecules called cytokines. Various different cytokines send different complex signals to other cells
including: calling immune system cells to the site of the infection, telling endothelial (blood vessel "lining") cells to let these other cells
through and telling immune system cells to activate themselves.
Cooperate with complementary B-cells to get them to clone themselves and to release antibodies
Clone themselves to increase the number of this type of T-cell.
Helper T-cells are strongly implicated in the process of demyelination in multiple sclerosis. They are dealt with more fully in the entry for
helper T-cells.
The third type of T-cell, the suppressor T-cell, is involved with suppressing an immune response. It is not well understood how they do but
they probably use several mechanisms including "programmed cell death" (apoptosis) which involves sending cytokines to other immune
system cells telling them to commit suicide. Suppressor T-cells are dealt with more fully in the entry for suppressor T-cells.
T-cells are manufactured in the bone marrow but migrate to an organ called the thymus where they are matured via a process called
affinity maturation which removes those which are active against the body's own antigens (autoreactive). Selection for particular T-cells is
dealt with in the entry on the thymus.
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