Dendritic cells are antigen presenting cells that play a key role in the immune system. Dendritic cells possess numerous long outgrowths called dendrites that give them a very large surface area of contact with their surroundings in the detection of pathogens such as bacteria and viruses. Dendritic cells have a very large contact surface area relative to their overall cell volume.
In primates, the dendritic cells are usually divided into two main groups: the myeloid dendritic cells (mDCs) and the plasmacytoid dendritic cells (pDCs).
Also called conventional dendritic cells, mDCs are similar to monocytes or white blood cells and are further subdivided into the more abundant mDC-1 which is a major stimulator of T cells and the less common mDC-2 that helps fight infections caused by wounds. The mDCs secrete the cytokine interleukin-12 and they display the Toll-like receptors (TLRs) TLR 2 and TLR 4.
The pDCs resemble plasma cells but share some features of mDCs. Previously known of as interferon-producing cells, these can secrete large amounts of interferon-alpha and they display TLR 7 and TLR 9.
To differentiate between the two types of dendritic cells, markers BDCA-2, BDCA-3, and BDCA-4 are used.
Another type of dendritic cell is the follicular dendritic cell or fDC. Unlike the myeloid and plasmacytoid forms, these dendritic cells are not of hematopoietic origin and do not express MHC class II. These fDCs are of mesenchymal origin but are located in lymphoid follicles and possess dendrites.
Dendritic cells cultured in the laboratory differ in several respects to those isolated form the body but are still used in research because they are more readily available. These include the Mo-DCs or MDDCs that are matured from monocytes and the HP-DCs that have been derived from hematopoietic progenitor cells.