Immunity and Infection

Immunity is the ability to resist infection. Infection is the invasion of the body by a harmful agent, called a pathogen - which means something that causes disease. There are lots of infectious agents - bacteria, viruses, fungi, rickettsia and protozoa - collectively called microbes, because they can only be seen using a microscope. Another name is germ - it means the same as microbe, or ‘micro-organism’. Our ability to defend ourselves against them depends on a number of factors: an intact skin is a great barrier; when it’s broken germs that otherwise would be harmless can cause illness. Once an infectious agent has entered the body, resistance depends on our immune system, which consists of white blood cells and circulating chemicals called antibodies. There are several different white blood cells which have different functions: neutrophils attack bacteria, lymphocytes attack viruses. These cells are made in the bone marrow, and they mature in different ways in different sites. Some of the lymphocytes stay in the bone marrow - called B-cells - where they make the antibodies, called gammaglobulins, directed against invading microbes. Some, in the foetus and the young child - up to puberty, in fact - are processed in the thymus gland (in the front of the chest at the bottom of the neck) and become T-cells. These migrate to the lymphatic tissue - the lymph nodes which are spread throughout the body (joined together by lymph channels), the spleen and the tonsils. In these tissues they lie in wait: when viruses appear some of the T-cells, called killer cells, wade in and attack, assisted by other lymphocytes called helper cells. Because the lymph nodes are making new T-cells they swell up, which is why we get swollen glands when we have an infection.

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