A second type of activity, cellular response, is also an essential component. The various and complex mechanisms of immunity are basic to the body's ability to protect itself against specific infectious agents and parasites, to accept or reject cells and tissues from other individuals, as in blood transfusions and organ transplants, and to protect against cancer, as when the immune system recognizes malignant cells as not-self and destroys them. In recent years there has been extensive research into the body's ability to differentiate between cells, organisms, and other substances that are self, and therefore not alien to the body, and those that are not-self and therefore must be eliminated. Both types of responses are instigated by lymphocytes that originate in the bone marrow as stem cells and later are converted into mature cells having specific properties and functions. There are several types of T cells involved in cell-mediated immunity, delayed hypersensitivity, the production of lymphokines, and the regulation of the immune response of other T and B cells.
Humans have no analogous organ.
B-lymphocytes mature into plasma cells that are primarily responsible for forming antibodies, thereby providing humoral immunity. At the time that a substance enters the body and is interpreted as foreign, antibodies are released from plasma cells and enter the body fluids where they can react with the specific antigens for which they were formed. Most of the B-lymphocytes activated by the presence of their specific antigen become plasma cells, which then synthesize and export antibodies. It is these memory cells that provide continued immunity after initial exposure to the antigens. The antibodies produced during this primary response are predominantly of the IgM class of immunoglobulins.