A special type of cell that is a key regulator of the immune system, acting as a professional antigen-presenting cell, APC, capable of activating naive T cells and stimulating the growth and differentiation of B cells. Dendritic cells are found, for example, in the lymph nodes and spleen. As an APC, a dendritic cell can retain antigen for long periods on its surface, present the antigen to a T or B cell and so influence their behavior.