Lymphocytes: The Specialists

Lymphocytes are specialized macrophages. Their purpose is to identify and destroy invading antigens. Lymphocytes all begin as stem cells in bone marrow and develop an antigen receptor as it matures. This is a structure on its surface that can bind with a matching structure on the antigen, much like a lock-and-key. Each individual lymphocyte can only have one receptor. When an antigen invades, only the lymphocytes whose receptors match with it are activated.

Lymphocytes continually circulate between the blood stream and the lymphoid organs. These organs include the lymph nodes, tonsils, spleen, and specialized areas of the lungs and intestines. By circulating in this manner, the body is continually monitored for invaders.

There are two places where lymphocytes can mature in. Those that remain in the bone marrow are B-lymphocytes, also known as B-cells. They make antibodies.

The other type of lymphocyte matures in the thymus organ, which is located under the breastbone. These are called T-lymphocytes, or T-cells. Some of these lymphocytes, called cytotoxic lymphocytes (also known as killer T-cells), directly destroy cells that are recognized as invaders. Helper T-cells, another type of T-lymphocyte, control all immune responses.