Dendritic cells are a bit like spies sitting in among other cells. If they detect pathogens (foreign substances) within the body, they will ingest some, and molecules of the pathogens, called antigens, appear on the surface of the dendritic cell. The dendritic cell then leaves the site of infection and moves to the nearest lymph node. It stays there for about a week, displaying the antigens to the T and B cells that move through the lymph node.
Acknowledgement: Prof Gareth Jones, Wellcome Images