Antigen-presenting cell

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An antigen-presenting cell (APC) 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). 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 naive T cell). These cells generally express MHC class II as well as MHC class I molecules, and can stimulate CD4+ ("helper") T 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.

[edit] Types

APCs fall into two categories: professional or non-professional. There are three main types of professional antigen-presenting cells:

These APCs are very efficient at phagocytosis, which allows them to present exogenous as well as internal antigens. For the purpose of effectively stimulating naive T cells, APCs possess co-stimulatory molecules: cell-surface molecules that deliver essential signals to T cells, allowing the T cells to become activated and mature into fully-functional forms.

Dendritic cells, and to a lesser extent macrophages, 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. B cells, which express antibody, can very efficiently present the antigen to which their antibody is directed, but are inefficient APC for most other antigens. 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:

[edit] 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.

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