The immune system guards the body against foreign, disease-producing substances. Its "workers" are various white blood cells. A lymphocyte is a specialized form of white blood cell, representing 25-40% of the total blood count, whose numbers increase during viral infection and when you’re fighting cancer.

Lymphocytes are produced in the bone marrow and come in two basic forms. B cells mature in the bone marrow and produce antibodies to neutralize foreign cells (this is known as humoral immunity). B cells account for 10-15% of all lymphocytes.

T cells mature in the thymus gland and react to and destroy specific invading antigens (this is known as cell-mediated immunity); 75-80% of lymphocytes are T cells. T cells are predisposed to respond to specific foreign substances (antigens) or infections.

Helper T cells (also known as T-4 or CD-4 cells) secrete immune proteins (particularly the interleukins and interferon) to stimulate B cells and macrophages, and activate Killer T cells; they account for 60-75% of T cells.

Killer T cells (T-8 or CD-8 cells) bind to the specific invader and secrete enzymes to destroy it; they account for 25-30% of T cells.

Suppressor T cells prevent excessive immune reactions by suppressing antibody activity.

Natural Killer (NK) cells are a type of nonspecific, free-ranging lymphocyte that is neither a B nor T cell. Unlike other lymphocytes, NK cells are not activated by a specific antigen—they can recognize and quickly destroy any antigen on first contact. They have potent cell-killing activity, being "armed" with an estimated 100 different biochemical poisons for destroying foreign cells.

As with antibodies, their role is surveillance, to rid the body of aberrant or foreign cells before they can grow and produce cancer or infection. NK cells account for 5% to 10% of all lymphocytes. An estimated 65% of normal healthy individuals have NK cell function between 30 and 160, 20% between 20 and 29, 10% between 161 and 250, and 5% have less than 20.