The immune system consists of cells and factors that work to protect an organism from antigens (foreign cells or substances). T lymphocytes (T cells) are one group of cells that play important roles in immunity. Dendritic cells are another important group of immune cells that process the antigens for T cells. Not only do dendritic cells help T cells to act upon antigens, they aid in T cells’ tolerance of the body’s own cells. They do this by helping the T cells to distinguish between invading organisms and the body’s cells and molecules, between self and nonself. This role of dendritic cells helps protect the body from autoimmune attack by its T cells. Therefore, dendritic cells play key and essential roles in the direction and control of appropriate T cell responses.