Antigen presentation is a process in the body's
immune systemAn immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumour cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
by which macrophages, dendritic cells and other cell types capture
antigenAn antigen is a substance that prompts the generation of antibodies and can cause an immune response. "Self" antigens are usually tolerated by the immune system; whereas "Non-self" antigens are identified as intruders and attacked by the immune system...
s and then enable their recognition by T-cells.
The basis of adaptive immunity lies in the capacity of immune cells to distinguish between the body's own cells, and infectious pathogens. The host’s cells express “self” antigens that identify them as such.
Antigen presentation is a process in the body's
immune systemAn immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumour cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...
by which macrophages, dendritic cells and other cell types capture
antigenAn antigen is a substance that prompts the generation of antibodies and can cause an immune response. "Self" antigens are usually tolerated by the immune system; whereas "Non-self" antigens are identified as intruders and attacked by the immune system...
s and then enable their recognition by T-cells.
The basis of adaptive immunity lies in the capacity of immune cells to distinguish between the body's own cells, and infectious pathogens.
The host’s cells express “self” antigens that identify them as such. These antigens are different from those in bacteria ("non-self" antigens) or in virally-infected host cells (“missing-self”). The ability of the
adaptive immune systemThe adaptive immune system is composed of highly specialized, systemic cells and processes that eliminate or prevent pathogenic challenges. Thought to have arisen in the first jawed vertebrates, the adaptive or "specific" immune system is activated by the “non-specific” and evolutionarily older...
to survey for infection requires specialized pathways of enabling recognition of pathogen-derived antigens by T cells.
Antigen recognition
Unlike B cells, T cells fail to recognize antigen in the absence of antigen-presentation, with the important exception of the
superantigenSuperantigens are a class of antigens which cause non-specific activation of T-cells resulting in polyclonal T cell activation and massive cytokine release...
s. The
T cell receptorThe T cell receptor or TCR is a molecule found on the surface of T lymphocytes that is, in general, responsible for recognizing antigens bound to major histocompatibility complex molecules....
is
restrictedMHC-restricted antigen recognition, or MHC restriction, refers to the fact that a given T cell will recognize a peptide antigen only when it is bound to a particular MHC molecule...
to recognizing antigenic peptides only when bound to appropriate molecules of the
major histocompatibility complexThe major histocompatibility complex is a large genomic region or gene family found in most vertebrates. It is the most gene-dense region of the mammalian genome and plays an important role in the immune system and autoimmunity. The diversity of MHC is important in the immune diversity in the...
(MHC), also known in humans as
Human leukocyte antigenThe human leukocyte antigen system is the name of the major histocompatibility complex in humans. The superlocus contains a large number of genes related to immune system function in humans. This group of genes resides on chromosome 6, and encode cell-surface antigen-presenting proteins and many...
(HLA)).
Most cells are capable of presenting antigen and activating the adaptive response. Some cells, however, are specially equipped to acquire and present antigen, and to prime naive T cells. Dendritic cells, B cells, and macrophages play a major role in the innate response, and also act as professional antigen presenting cells (APC). These professional APCs are equipped with special immunostimulatory receptors that allow for enhanced activation of T cells.
Several different types of T cell can be activated by professional APCs, and each type of T cell is specially equipped to deal with different pathogens, whether the pathogen is bacterial, viral or a toxin. The type of T cell activated, and therefore the type of response generated, depends, in part, on the context in which the antigen was first encountered by the APC.
Intracellular antigens: Class I
Intracellular antigens are mainly produced by viruses replicating within a host cell, though antigens here can also derive from cytoplasmic bacteria or the host cell's own proteins. The host cell digests cytoplasmic proteins by a specialized enzyme complex, the
proteasomeProteasomes are large protein complexes inside all eukaryotes and archaea, as well as in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
into small
peptidePeptides are short polymers formed from the linking, in a defined order, of α-amino acids. The link between one amino acid residue and the next is called an amide bond or a peptide bond....
s. A specialized carrier, the Transporter associated with Antigen Processing (TAP) complex moves the peptide into the
endoplasmic reticulumThe endoplasmic reticulum is an eukaryotic organelle that forms an interconnected network of tubules, vesicles, and cisternae within cells. The lacey membranes of the endoplasmic reticulum were first seen by Keith R. Porter, Albert Claude, and Ernest F...
, allowing the antigenic peptide to be coupled to an
MHC Class IThe major histocompatibility complex is a large genomic region or gene family found in most vertebrates. It is the most gene-dense region of the mammalian genome and plays an important role in the immune system and autoimmunity. The diversity of MHC is important in the immune diversity in the...
molecule and transported to the cell surface.
MHC Class I molecules present antigen to CD8+
cytotoxic T cellA cytotoxic T cell belongs to a sub-group of T lymphocytes that are capable of inducing the death of infected somatic or tumor cells; they kill cells that are infected with viruses , or are otherwise damaged or dysfunctional...
s. With the exception of some cell types (such as erythrocytes), Class I MHC is expressed by almost all host cells. Cytotoxic T cells (also known as T
C, killer T cell, or cytotoxic T-lymphocyte (CTL)) are a population of T cells which are specialized for inducing the death of other cells. Recognition of antigenic peptides through Class I by CTLs leads to the killing of the target cell, which is infected by virus, intracytoplasmic bacterium, or are otherwise damaged or dysfunctional.
Extracellular antigens: Class II
Dendritic cells (DCs) phagocytose exogenous pathogens, such as bacteria, parasites or toxins in the tissues and then migrate, via chemotactic signals, to
T cell'T cells' belong to a group of white blood cells known as lymphocytes, and play a central role in cell-mediated immunity. They can be distinguished from other lymphocyte types, such as B cells and natural killer cells by the presence of a special receptor on their cell surface called T cell receptors...
enriched lymph nodes. During migration, DCs undergo a process of maturation in which they lose phagocytic capacity and develop an increased ability to communicate with T-cells in the lymph nodes. This maturation process is dependent on signaling from other pathogen-associated molecular pattern (PAMP) molecules through
pattern recognition receptorPattern recognition receptors, or PRRs, are proteins expressed by cells of the innate immune system to identify pathogen-associated molecular patterns, or PAMPs, which are associated with microbial pathogens or cellular stress...
s, such as the members of the
Toll-like receptorToll-like receptors are a class of proteins that play a key role in the innate immune system. They are single membrane-spanning non-catalytic receptors that recognize structurally conserved molecules derived from microbes...
family.
The DC uses
lysosomethumb|350px|Schematic of typical animal cell, showing subcellular components. [[Organelle]]s:
[[nucleolus]]
[[cell nucleus|nucleus]]
[[ribosomes]]
[[vesicle |vesicle]]
...
-associated
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process are called substrates, and the enzyme converts them into different molecules, called the products. Almost all processes in a biological cell need enzymes to occur at...
s to digest pathogen-associated proteins into smaller peptides. In the lymph node, the DC will display these antigenic peptides on its surface by coupling them to MHC Class II molecules. This MHC:antigen complex is then recognized by T cells passing through the lymph node. Exogenous antigens are usually displayed on
MHC Class IIMHC Class II molecules are found only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are professional antigen-presenting cells ....
molecules, which interact with CD4
+ helper T cells. CD4
+ lymphocytes, or T
H, are immune response mediators, and play an important role in establishing and maximizing the capabilities of the adaptive immune response.
Expression of Class II is more restricted than Class I. High levels of Class II are found on
dendritic cellDendritic cells are immune cells that form part of the mammalian immune system. Their main function is to process antigen material and present it on the surface to other cells of the immune system, thus functioning as antigen-presenting cells....
s, but can also be observed on activated macrophages, B cells, and several other host cell types in inflammatory conditions.
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