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Antigen Presenting Cells
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Antigen-presenting cells (APC) are specializedwhite blood cells that displayforeign antigen complexes with major
histocompatibility complex (MHC) on theirsurfaces.
T-cells may recognize these complexes usingtheir T-cell receptors (TCRs).
These cells process antigens and present themto T-cells.
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http://en.wikipedia.org/wiki/Antigenhttp://en.wikipedia.org/wiki/Major_histocompatibility_complexhttp://en.wikipedia.org/wiki/Major_histocompatibility_complexhttp://en.wikipedia.org/wiki/T-cellhttp://en.wikipedia.org/wiki/T-cell_receptorhttp://en.wikipedia.org/wiki/Antigen_processinghttp://en.wikipedia.org/wiki/Antigen_presentationhttp://en.wikipedia.org/wiki/Antigen_presentationhttp://en.wikipedia.org/wiki/Antigen_processinghttp://en.wikipedia.org/wiki/T-cell_receptorhttp://en.wikipedia.org/wiki/T-cell_receptorhttp://en.wikipedia.org/wiki/T-cell_receptorhttp://en.wikipedia.org/wiki/T-cellhttp://en.wikipedia.org/wiki/T-cellhttp://en.wikipedia.org/wiki/T-cellhttp://en.wikipedia.org/wiki/Major_histocompatibility_complexhttp://en.wikipedia.org/wiki/Major_histocompatibility_complexhttp://en.wikipedia.org/wiki/Antigen7/31/2019 Antigen Presenting Cell
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Mode of action
First, the APC engulfs the antigen.
Enzymes inside the APC break down the
antigen into smaller particles.
The processed antigens are transported to
the surface of the APC, bound with either an
MHC (major histocompatibility complex) class
I or class II molecule.
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Antigen processing
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This complex forms epitopes (part of a foreign
substance that can be recognized by the
immune system), which the T-cell receptor
(TCR) recognizes and binds to.
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Types
The APCs are divided into two categories
professional and
non-professional APCs.
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Professional APCs
Professional APCs express MHC class II .
Only professional APCs are able to activate ahelper T-cell that has never encountered its
antigen before. There are three main types of professional
antigen-presenting cells
macrophages,
dendritic cells and
B-cells.
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Antigen
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These cells are able to engulf the antigen
quickly during a process called phagocytosis.
Once the T-cell recognizes and binds to the
MHC molecule complex, the APC sends out an
additional co-stimulatory signal to activate the
T-cell.
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non-professional APCs
non-professional APCs express MHC class I Non-professional APCs include
fibroblasts (skin cells),
thymic epithelial cells,
thyroid epithelial cells,
glial cells (brain cells),
pancreatic beta cells and
vascular endothelial cells.
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While almost every cell in the body is
technically an APC because they can present
antigens to T-cells,
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Macrophages
Macrophages are white blood cells that are
found inside the tissues of all vertebrates.
These cells originate from monocytes in the
bone marrow.
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Macrophages are phagocytes, which means
they are able to engulf antigens that enter the
body.
The lifespan of macrophages ranges from
months to years, and they can digest more
than 100 bacteria before they die.
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Macrophages are in a resting state until they
are activated by an immune response.
Upon activation, these cells travel toward the
site of injury, and they engulf pathogens.
Once a macrophage ingests a pathogen, the
pathogen is trapped inside the cell's food
vacuole, which then fuses with a lysosome.
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Enzymes and toxic peroxides inside the cell
start to ingest the foreign substance.
The cells then secrete interferons, lysozyme
and other factors that stimulate lymphocytes
and other immune cells to respond to the
antigens.
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Dendritic Cells
Dendritic cells are present in small amounts inthe body tissues that frequently come intocontact with the external environment.
They are present in the skin (where they areoften called langerhans cells) and the innerlining of the nose, stomach, lungs andintestines.
These cells have branched projections, calleddendrites.
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Immature dendritic cells (also called veiled
cells) are found in the bloodstream.
Immature dendritic cells are characterized by
high endocytic activity and low T-cell
activation potential.
Their pattern recognition receptors (PRRs) are
constantly sampling their surroundings for
pathogens, such as bacteria and viruses.
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Once the dendritic cell comes into contact
with a pathogen, they are activated to engulf
the invading substance.
Once the antigen is engulfed, the dendritic
cell matures and combines the broken down
antigen with MHC.
This complex is then presented to T-cells on
its cell surface.
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B-Cells
Unlike the other two APCs, they produce
antibodies (immunoglobulin) that are specific
to certain antigens.
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B-cells are continually produced in the bone
marrow.
Once the B-cell reaches maturity, it can
express both IgM and IgD on the cell surface.
This mature cell is now able to respond to
antigens.
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Once the immunoglobulin molecule interacts
with an antigen, the B-cell becomes activated
and begins to divide and differentiate into
many antibody-producing cells (plasma cells).
Each plasma cell secretes millions of identical
antibody molecules, which are released into
the bloodstream.
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