The immune system is the body’s defense force. It protects against infections that threaten to enter through the skin, the mouth, ears, nose, throat and lungs, the intestinal tract, and the openings of the urinary and genital organs.
The potential invaders include viruses, bacteria, fungi, small parasites (such as those causing malaria and sleeping sickness), and larger parasites ranging from mites (such as scabies) to a whole host of worms and flukes.
The immune system also repairs wounds and defends (often successfully) against cancers. The biological defense force, like a nation’s military defenses, includes a whole range of specialist units designed to deal with threats of various kinds.
The two basic immune defenses are cells and chemicals.
- The best-known action of defense cells is phagocytosis: the swallowing, engulfing, and killing of a microorganism by a defense cell.
- The best-known chemical defense is the antibody: a protein, tailor-made by immune cells to correspond with chemical features on the surface of an invading microorganism, fitting like a key into a lock.
Once coated with antibody, the virus or bacterium becomes easier to destroy by phagocytosis. So how do the invaders ever win? Part of the answer is that the chemical defenses take some time to become effective.
When the body is infected for the first time by a particular microorganism, there is a race between the multiplying microbes and the multiplying immune cells producing antibodies. Given time, the body usually wins; eventually, enough antibodies are formed to overwhelm the invaders.
But if the initial invasion force is large, or the immune system is weak, the battle may be lost. Immunity may be impaired through old age, drugs such as steroids, and diseases that affect the immune system – of which the most important is AIDS.
The immune system’s cells and chemicals are mostly found in the lymph glands, the raisin-sized lumps that become obviously swollen beneath the skin of the neck or beneath the arm during an infection. Lymph glands are found in all parts of the body and are linked together by a network of lymph vessels, which are similar to small veins but thinner.
The tonsils, the spleen, and the thymus (a gland behind the breastbone) are all part of the immune system. The lymph vessels join up to form the thoracic duct, which itself joins up with the blood circulation.
The immune cells and chemicals are in constant movement around the body. They travel in the blood, passing out into the body tissues through the smallest blood vessels, the capillaries. Here they can get to grips with infecting bacteria.
Cells and fluid drain from the tissues into the lymph vessels and are taken first to lymph glands, where micro-organisms and any particles of non-living material that get into the body are trapped. Eventually, the lymphatic fluid and cells return to the bloodstream and resume their circulation.
The body’s immune system is on constant alert to recognize any foreign invasion. The immune system recognizes not just bacteria and viruses as “foreign”, but also healthy tissues from another human being – witness the frequent rejection of transplanted organs.
Sometimes this discrimination goes wrong the body’s immune system attacks part of itself, and the result is a group of disorders known as autoimmune diseases. There are two main groups.
The first group affects a single organ, such as the thyroid gland, the stomach, the adrenal glands, or the cells of the pancreas that make insulin. The second group affects particular parts of cells or tissues that are present in more than one organ, such as small blood vessels.
Autoimmune diseases run in families. But a genetic predisposition alone is not sufficient – some environmental trigger is required. Familiar autoimmune disorders are:
- The systemic lupus erythematosus (SLE): In SLE, antibodies are found to DNA (anti-DNA), so the disease damages many organs. SLE is 10 times as common in women as men. A rash, typically across the cheeks, and painful joints are the two commonest symptoms, but SLE may also affect the kidneys, the heart, and the brain.
- Rheumatoid arthritis: Cells of the immune system turn against the lining of joints and tendons, destroying them. The process becomes self-perpetuating and the joints and tendons of the body become chronically inflamed, painful, and deformed if the process persists. Primary biliary cirrhosis (PBC) is mainly a disorder of middle-aged women. Destruction of the bile ducts in the liver leads to disruption to the flow of bile, and jaundice.
- Immunodeficiency: The immune system can also be under-active, and fail to deal appropriately with foreign invaders. This makes the person prone to infection. Some causes of immunodeficiency are genetically inherited, resulting in too few lymphocytes or loo little antibody being produced. Malnutrition can cause immunodeficiency, as can infections, such as measles, glandular fever, and AIDS (acquired immune deficiency syndrome).