When they talk about DC migration. The DC are present in the body after they're produced by cells in the bone marrow, and they rest somewhere, at strategic places, for a pathogen, once the pathogen enters they bind to the pathogen, and they become one of the APC (antigen presenting cell, like macrophages), they go up into the lymphatics until a T cell sees them in a lymph node for example, the T cell will see the antigen and it starts splitting into effector cells. That's the migration. That's why they form the bridge between innate and adaptive immunity, the lymphocyte effector and memory cells are the activated adapative immune system.