Antigen-presenting cell

AnAntigen-presenting cell (APC) OrAccessory cellIs a cell that displays foreign antigens complexed with major histocompatibility complexes (MHCs) on their surfaces; this process is known as antigen presentation. t-cells may recognize these complexes using their T-cell factors (TCRs ). these cells processantigens and present them to T-cells.

 

 

Antigen presentation stimulates T cells to become either "cytotoxic" CD8 + cells or "helper" CD4 + cells.

 

 

Contents [hide]

• 1 Types
  • 1.1 Professional APCs
  • 1.2 Non-professional
• 2 Interaction with T cells
• 3 External links
• 4 References

 

Types [edit]

APCs fall into two categories: professional or non-professional.

T cells cannot recognize, and therefore cannot respond to, 'free' antigen. T cells can only 'access' an antigen that has been processed and presented by cells via carrier molecules like MHCs and CD1 molecules. most cells in the body can present antigen to CD8 + T cells via mhclass I molecules and, thus, act as "APCs"; however, the term is often limited to specialized cells that can prime T cells (I. e ., activate a T cell that has not been exposed to antigen, termedNaive T cell). These cells, in general, express mhclass II as well as mhclass I molecules, and can stimulate CD4 + ("helper") T cells as well as CD8 + ("cytotoxic ") T cells, respectively.

To help distinguish between the two types of APCs, those that express mhclass II molecules are often calledProfessional antigen-presenting cells.

Professional APCs [edit]

Professional APCs are very efficient at internalizing antigen, either by phagcytosis or by receptor-mediated endocytosis, and then displaying a fragment of the antigen, bound to a class II mhmolecule, on their membrane. the T cell recognizes and interacts with the antigen-class II mhmolecule complex on the membrane of the antigen-presenting cell. an additional co-stimulatory signal is then produced by the antigen-presenting cell, leading to activation of the T cell. the expression of co-stimulatory molecules is a defining feature of professional APCs.

There are three main types of professional antigen-presenting cells:

• Dendritic cells (DCs), which have the broadest range of antigen presentation, and are probably the most important APC. activated DCs are especially potent TH cell activators because, as part of their composition, they express co-stimulatory molecules such as B7. This B7 co-stimulator of mature interdigitating dendritic cell (IDC) interacts with surface CD2 of na & iuml; ve T-cell.
• Macrophages, which are also CD4 + cells and are therefore susceptible to infection by HIVas well, since HIV invades immune cells through CD4 + receptor interactions.
• Certain B-cells, which express (as B cell receptor) and secrete a specific antibody, can internalize the antigen, which bind to its CAA and present it inmarshated to mhii molecule, but are inefficient APC for most other antigens.
• Certain activated epithelial cells

Non-professional [edit]

A non-professional APC does not constitutively express the Major Histocompatibility Complex class II (mhclass II) proteins required for interaction with naive T cells; these are expressed only upon stimulation of the non-professional APC by certain cytokines such ASFN-& gamma ;. non-professional APCs include:

• Fibroblasts (skin)
• Thymic epithelial cells
• Thyroid epithelial cells
• Glial cells (brain)
• Pancreatic beta cells
• Vascular endothelial cells

Interaction with T cells [edit]

After APCs have phagcytosed pathogens, they usually migrate to the vast networks of lymph networks and are carried via lymph flow to the draining lymph nodes (this network is collectively known as the distributed system ). the lymph nodes become a collection point to which APCs such as dendritic cells (DCs) can interact with T cells. they do this bychemotaxis, which involves interacting with chemokines that are expressed on the surface of cells (e.g ., endothelial cells of the high endothelial venules) or have been released as chemical messengers to draw the APCs to the lymph nodes. during the migration, DCs undergo a process of maturation; in essence, they lose most of their ability to further engulf pathogens, and they develop an increased ability to communicate with T cells. enzymes within the cell digest the swallowed pathogen into smaller pieces containing epitopes, which are then presented to T cells using MHCs.

Recent research indicates that only certain epitopes of a pathogen are presented because they are immunodominant, it seems as a function of their binding affinity to the MHCs. the stronger binding affinity allows the complex to remain kinetically stable long enough to be recognized by T cells.

External links [edit]

• Antigen: protease degradation on YouTube-PMAP The proteolympus SIS Map-animation
• Antigen-Presenting Cells at the US National Library of Medicine Medical Subject Headings (MeSH)

Antigen-presenting cell