Lymphocyte (pink) getting activated by a dendritic cell (blue).

Dendritic cells are immune cells and form part of the mammal immune system. They are present in those tissues which are in contact with the environment: The skin and the lining of nose, lungs, stomach and intestines. They have long spiky arms, called dendrites, whence the name.

Dendritic cells constantly sample the surroundings for viruses and bacteria. Once they have captured such an invader, they cut its proteins into small pieces and present those fragments at their cell surface using MHC molecules.

They then travel through the blood stream to the spleen or through the lymphatic system to a lymph node. Here they act as antigen presenting cells: They activate helper T-cells and killer T-cells as well as B-cells by presenting them with the pieces of the invader. Depending on the type of invader, this results in an immune response involving antibodies or killer cells.

Every helper T-cell is specific to one particular antigen. Only dendritic cells are able to activate a helper T-cell which has never encountered its antigen before.

Image by Olivier Schwartz