The thymus is the major gland of our immune system. The thymus is responsible for many immune system functions including the production of T-lymphocytes, a type of white blood cell responsible for cell mediated immunity. Cell mediated immunity is a type of immunity in which specialized cells carry' out defensive activities. They protect the body against pathogens including the protists and fungi which have entered the host's cells. T-cells and B-cells are the type of lymphocytes that develop from bone marrow cells. Those lymphocytes that migrate to the thymus and differentiate are called T-cells and those cells that continue to be in the bone marrow for differentiation are known as B-cells. T-cells are responsible for cell mediated immunity, however, B- cells produce antibodies and take part in antibody mediated immunity.