The fundamental task of the immune system is to defend "self" from "nonself." Lymphocytes are the primary cells of the immune system that developed one of the most sophisticated and comprehensive defense mechanisms in the biological system. T cells play a central role in orchestrating the immune response. Further, they are instrumental in eliminating intracellular pathogens (viruses, some bacteria) through the generation of cytotoxic T cells. B cells defend against extracellular pathogens by producing antibodies. Natural killer cells are an important component of innate immunity. Dendritic cells play a key role in initiating the immune response by presenting foreign antigens to T cells. The interaction among T cells, B cells, dendritic cells, and natural killer cells constitute the fundamental defense network of the host. The failure of any of these components severely jeopardizes the integrity of the immune system and its ability to mount the most appropriate immune response.