The immune system

Sometimes the immune system overreacts, or attacks the wrong target.

Allergy is an exaggerated response of the immune system to certain substances that are harmless to most people: allergens, such as dust mites, pollen and animal dander (skin flakes and fur). The antibodies involved are of the IgE type. They make mast cells release chemicals such as histamine and serotonin, which cause sneezing, watery eyes, a running nose and difficulty in breathing. Drugs such as anti-histamines, adrenalin and steroids reduce the symptoms. A person may sneeze only in a new place because the allergens there are different. Modern, protected lifestyles seem to have made allergies more common.

Auto-immunity. The immune system normally tells self from non-self. Sometimes, through genetic or other causes, it attacks the body's own cells. This is an auto-immune disease; rheumatoid arthritis, which damages the joints, is an example.

The immune system is made up of lymphoid organs and tissues, where lymphocytes form, mature and meet antigens.

• Primary lymphoid organs: where lymphocytes form and mature. The bone marrow makes all blood cells, including lymphocytes, and B-cells mature there. The thymus, a gland near the heart, is where T-cells mature. It is large at birth and shrinks with age, becoming very small by puberty.
• Secondary lymphoid organs: where mature lymphocytes meet antigens and multiply. They include the spleen (which also filters the blood and stores red blood cells), the lymph nodes (which trap microbes carried in the lymph), the tonsils, Peyer's patches of the small intestine, and the appendix.

Lymphoid tissue also lines the moist surfaces of the respiratory, digestive and urogenital tracts: the mucosa-associated lymphoid tissue (MALT). It makes up about 50% of all the lymphoid tissue in the body.