The Immune System

The Body has an impressive array of defenses to block, trap, and kill outside organisms it considers a threat, with a memory to prevent them attacking again. Without this defense system (known as the immune system) most people would constantly fall ill from the wide range of threats to the body. Like an army on constant full alert, the immune system is active night and day to guard against attack and to repel or destroy invaders.

The immune system consists of the blood supply, the lymph system, and small sets of organs known as the tonsils, thymus gland, and the spleen. Defense against disease is essentially a function of white cells in the blood (leukocytes), and it is one of the jobs of this group of organs to produce these cells.

White blood cells produce substances that defend the body against attack from anything harmful such as a bacterium or virus. If an invader (antigen) gets past the barrier of skin, hairs, and body fluid, the defending substances - neutrophils, basophils eosinophils, and, finally, macrophages - close in and finish off the organism. Some white cells travel in the bloodstream to fight infection while others remain in the lymph "nodes" (glands), situated mainly in the neck, armpit, and groin, where they destroy organisms brought to them by the lymph fluid. It is this battle between white cells and invading organisms that causes lymph nodes to become inflamed and swell; they can often be felt through the skin.

Body cells that become infected produce a wide range of chemical "messengers" to help in the fight against invaders. These chemicals (cytokines) - the main examples of which are interleukins and interferon - send messages to other cells telling them to protect themselves and thus prevent the spread of the infection. Complement, which also travels in the blood when an immune response is activated, coats the invading microbe to attract macrophages to destroy it.

Next in line of defense are T-cells and B-cells, both based in lymph tissue such as the spleen and lymph nodes.

T-cells are divided into "killers" (which destroy microbes), "helpers" (which stimulate the formation of antibodies), "suppressors" (which keep this production in check), and "memory" cells (which ensure a swift response should invaders of the same type return). B-cells produce antibodies, each of which is a response to a specific antigen. These not only protect the body but they also remember the antigen should another return in the future and try to get past the same defenses.