The immune system defends the body from invading organisms that may cause disease. One part of the immune system uses barriers to protect the body from foreign substances. These barriers include the skin and the mucous membranes, which line all body cavities; and protective chemicals, such as enzymes in saliva and tears that destroy bacteria.
Another part of the immune system uses lymphocytes, specialized white blood cells that respond to specific types of foreign invaders. B lymphocytes produce proteins called antibodies, which circulate in the blood and attack specific disease-causing organisms. T lymphocytes attack invading organisms directly.
Tonsils are masses of lymphoid tissue forming a ring around the walls of the pharynx, or throat. The lymphoid cells in the tonsils help protect the pharynx from invasion by disease-producing bacteria.
Some lymphocytes form in the bone marrow and then travel to the thymus gland, where they mature into T lymphocytes.
Lymph nodes are masses of tissue that attract lymphocytes and deploy them to areas of the body under attack by infectious agents.
The spleen is one of the lymphoid organs. Mature lymphocytes constantly travel through the blood to the lymphoid organs and then back to the blood again. This recirculation ensures that the body is continuously monitored for invading substances. Among its many functions, the spleen produces antibodies against various disease organisms and removes worn-out red blood cells from the bloodstream.
The bone marrow is the soft substance found in the center of some bones. All lymphocytes originate in the bone marrow. Those that mature in the bone marrow develop into B lymphocytes.
Lymphocytes travel throughout the body in the blood, but they often migrate into lymphatic vessels, which are found in all parts of the body except the brain. Lymphocytes travel within these vessels in a pale,
fat-laden liquid known as lymph.