Such immune defenses are brought about by a complex array of circulating cells called lymphocytes and monocytes (which are collectively known as white blood cells; cyte is a term for cells). There are two classes of lymphocytes: T cells and B cells. Both originate in the bone marrow, but T cells migrate to mature in the thymus (hence the T), while B cells mature in the bone marrow. B cells principally produce antibodies, but there are several kinds of T cells (T helper and T suppressor cells, cytotoxic killer cells, and so on).
