Special white blood cells called lymphocytes play a key role in the immune system's response to foreign invaders. There are two main groups, both of which form in bone marrow.

One group, called T-lymphocytes or T-cells, migrates to a gland called the thymus.

Influenced by hormones, they mature there into several types of cells, including helper, killer, and suppressor cells. These different types work together to attack foreign invaders. They provide what's called cell-mediated immunity, which can become deficient in persons with HIV, the virus that causes AIDS. HIV attacks and destroys helper T cells.

The other group of lymphocytes are called B-lymphocytes or B cells. They mature in the bone marrow and gain the ability to recognize specific foreign invaders.

Mature B cells migrate through the body fluids to the lymph nodes, spleen, and blood. In Latin, body fluids were known as humors. So B-cells provide what's known as humoral immunity. B-cells and T-cells both circulate freely in blood and lymph, searching for foreign invaders.