The immune system is the collective name given to various mechanisms that enable the body to defend itself against invasion by infection-carrying agents such as bacteria, viruses, fungi, and foreign proteins. The skin, certain glands, and the lymphatic system are all part of the immune system.
How the system works
Any invading organism trying to enter the body must break down the defensive barriers erected by the immune system. It will probably pass through an area of broken skin or enter the respiratory and digestive tract via the mouth or nose. There are glands along these routes that produce protective enzymes or natural antibiotic secretions, but if an organism does break through into body tissue, the immune system relies heavily on the lymphatic system.
The lymphatic system
This body system consists of lymph vessels that carry lymph from the tissue spaces between cells all over the body to lymph glands, or nodes. Lymph is a clear fluid that derives from plasma in blood from which proteins and other nutrients have been removed. Lymph drains from the bloodstream into capillaries and then into the lymph vessels. The flow of lymph is controlled by muscle contractions and valves. Invading organisms are trapped in lymph glands dotted throughout the lymphatic system. Here they are attacked by scavenging white blood cells called macrophages. Lymph glands also produce lymphocytes, another form of white blood cell. Some lymphocytes (B-cells) produce antibodies that attack antigens on the surface of an invading organism (see page 206); other lymphocytes (T-cells) destroy invading organisms directly. Lymphocytes have the ability to remember invaders that they have come across before, and so respond quickly if the body is invaded again. On the other hand, the immune system can fail to spot invaders, or it can overreact— by developing allergies—or react abnormally to the body’s own tissues, as in autoimmune diseases such as rheumatoid arthritis (see page 197). Alternatively, it may fail to recognize that its own cells have begun to function abnormally, hence the development of cancer.