The adaptive immune system can be subdivided into cell-mediated immunity, carried out by T cells, and humoral immunity, carried out by B cells. T cells expressing the CD4 T cell co-receptor are known as T helper cells and play a critical role in the activation and maturation of monocytes, cytotoxic T cells and B cells. T cells expressing the CD8 T cell co-receptor are known as cytotoxic T cells and eliminate host cells infected with intracellular pathogens as well as tumor cells. B cells mature into plasma cells that produce antibodies, also known as immunoglobulins (Ig), to eliminate extracellular microorganisms and prevent the spread of infection. The adaptive immune response can be distinguished from innate immunity by the capability of generating immunological memory, or protective immunity against recurring disease caused by the same pathogen (Janeway 2008).
