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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/Antigen

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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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