dendritic cell

Cells of the immune system, probably of myeloid origin, though some evidence suggest at least some types may be derived from lymphoid precursors.

Discovered by Ralph Steinman, these cells are exquisite antigen presenting cells (APC). They are able to stimulate T-cells to a much greater extent (given similar doses of antigen) than other APCs.

The current model of how dendritic cells, or DC, function in T-cell responses:

1. Immature DC encounters antigen, takes up antigen through phagocytosis and/or pinocytosis.
2. As the DC matures, it moves to the lymph node.
3. In it's mature state, DC do not take up additional antigen, and have high levels of MHC molecules and co-stimulators on their surface.
4. In the lymph node, T-cells are activated by the mature DC, proliferate, exit the lymph node, and traffic to the site of infection, presumably where the DC originally came from.

The discovery of the DC has somewhat marginalized the function of the macrophage, which was previously thought to be the master APC. Now, it seems that DC are most important in the initiation of T-cell-mediated immune responses, and that the macrophage functions at sites of infection.

Dendritic cells also mediate a response because naive cytotoxic T lymphocytes do not display MHC-II molecules. An activated helper T-lymphocyte attaches it's TCR to the MHC-II of the dendritic call and this also involves the CD4 co-receptor. The CD4 co-receptor is necessary because even the license to kill needs a license to kill.

The Dendritic cell now has an activated helper t-lymphocyte attached. The MHC-I of the dendritic cell can now attach to the TCR of a Cytotoxic T lymphocyte along with a CD8 co-receptor.

The activated cytotoxic T lymphocyte seeks out virus infected cells and attaches its TCR to their MHC-1 along with the CD8 co-receptor. A release of cytotoxins lyses the infected cell and it is no more a threat.

This may seem rather boring when all I mention is how the helper T lymphocyte uses the dendritic cell to activate a cytotoxic T lymphocyte, but currently this is undergoing more research and is rather interesting. There aren't any really good reasons for the use of a dendritic cell for this purpose. Like I mentioned earler, immunology isn't like James Bond having a license to kill. A license to kill requires a license to kill, and requires another license to kill.