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Human body

Immune system

Salmonella bacteria invading human cellsSalmonella bacteria invading human cellsThe immune system protects the body from invasion by pathogens: viruses or micro-organisms (also called microbes) such as bacteriafungi and parasites that causes disease. Toxins (poisonous chemicals) produced by microbes are also pathogens. The immune system is spread throughout the body and involves many different cells, tissues, organs and substances. When the immune system encounters a pathogen, it mounts what is called an immune response: a carefully co-ordinated attack on the invaders. The immune system can be divided into two categories: innate immunity, which you are born with, and adaptive immunity, which develops as the body becomes exposed to microbes over time. The two parts of the system work together to keep you healthy.


Platelets making the blood clotPlatelets making the blood clot

Innate immunity

We are all born with some degree of immunity. This innate immunity will attack invaders from when we are born. This part of the immune system—the first line of defence against pathogens—consists of barriers, such as the skin and mucous membranes of the nose and throat, which stop the pathogens from entering your body. These barriers tackle all types of pathogen and so are known as non-specific barriers. They can be either physical or chemical. 

T cells develop in the thymus, a small organ situated in front of the windpipe (trachea) and behind the breastbone. The thymus is so-called because it is shaped like the leaf of the thyme plant, a type of herb.

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