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An antigen-presenting cell (APC) or accessory cell is a cell that displays foreign antigen complexed with MHC on its surface. T-cells may recognize this complex using their T-cell receptor (TCR).
Additional recommended knowledge
APCs fall into two categories: professional or non-professional.
Although almost every cell in the body is technically an APC, since it can present antigen to CD8+ T cells via MHC class I molecules, the term is often limited to those specialized cells that can prime T cells (i.e., activate a T cell that has not been exposed to antigen, termed a naive T cell). These cells generally express MHC class II as well as MHC class I molecules, and can stimulate CD4+ ("helper") cells as well as CD8+ ("cytotoxic") T cells.
To help distinguish between the two types of APCs, those that express MHC class II molecules are often called professional antigen-presenting cells.
These professional APCs are very efficient at internalizing antigen, either by phagocytosis or by endocytosis, and then display a fragment of the antigen, bound to a class II MHC molecule, on their membrane. The T cell recognizes and interacts with the antigen-class II MHC molecule 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.
There are three main types of professional antigen-presenting cells:
As well, there are specialized cells in particular organs (e.g., microglia in the brain, Kupffer cells in the liver) derived from macrophages that are also effective APCs.
A non-professional APC does not constitutively express the Major histocompatibility complex proteins required for interaction with naive T cells; these are only expressed upon stimulation of the non-professional APC by certain cytokines such as IFN-γ. Non-professional APCs include:
Interaction with T cells
After dendritic cells or macrophages swallow pathogens, they usually migrate to the lymph nodes, where most T cells are. They do this chemotactically: chemokines that flow in the blood and lymph vessels "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 swallow 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 MHC.
Recent research indicates that only certain epitopes of a pathogen are presented because they are immunodominant, possibly as a function of their binding affinity to the MHC. The stronger binding affinity allows the complex to remain kinetically stable long enough to be recognized by T cells.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Antigen-presenting_cell". A list of authors is available in Wikipedia.|