Dendritic cells are cells that are found on all tissues exposed to an external environment (the skin, stomach, intestines, upper respiratory tract and lungs) that mediate reactions with the immune system. Immature dentritic cells are also found in the blood. They are created by the bone marrow. When activated by the chemistry of a foreign body such as a pathogen, the cells migrate through the blood to the lymphatic system where they activate T cells and B-cells. They are named for tree-like projections, dentrites, that appear during certain stages of the cell's development. This gives the cell a very high amount of surface area compared to its volume.
Dendrites were first described by Paul Langerhans in the late nineteenth century and the dendritic cells in the skin are named after him (as in Langerhans cell histiocytosis).
Some pathogens, such as HIV, use dentritic cells to their advantage. HIV attaches itself to dendrites which carry it to the lymphatic system where the virus can infect T cells. They also play a role in many autoimmune diseases including allergies, lupus and Crohn's disease.